I have 3 NPCs, a Book Merchant, a Food Merchant, and a Bartender (community lot) that are losing motives, even though they should be locked. I do not have SP installed and I do not have my mega mall in the town gumming up the works (this is a completely different game from the one that did have it, so it isnt residual effects from that either). This time I have an error log file for one of them, though. All three of them actually threw an error yesterday (real world time), but I didnt know their motives were borked until today when I visited the lot, so I only have one of their error logs. I am uploading the log to see if the error and their motives dropping are related

Never encountered this bug before. Did you try resetsim. Usually fixes stuff. Did you have a mod that you removed? Maybe Nraas Register can help.

The OP has Register, v80.

The script error is not related. The error is from this sim having a book stuck to their hand, this kind of thing happens to a lot of sims in many players' games. That raises the question of why they were using the Read Something interaction in the first place if they should have been tending their cash register, but I also do see the ones who aren't Workaholics wander away from their stations and then get pulled back eventually.

Not really relevant I guess, but just last week my two cash register attendants on one lot (Cupcake register that I later replaced with a Generic Role Sim from Arsil's mod and a coffee barista) spent sim hours having a snowball fight out back. That wasn't a big help for the customers, but they looked like they were having fun so I left them alone.

In my experience, Role Sims' motives are not usually locked in and only SP's Static Motives can do that. Their motives wander downwards and are occasionally replenished back to +100 when they are on duty. I rarely see them get to the point where they are actually suffering from hunger, ready to pee on the floor (usually they run for the nearest bathroom instead), or passing out from exhaustion, and maybe this isn't supposed to be this way, but it does happen sometimes. Not every day, though.

I will note that your sim is also Absent-Minded. Maybe that's not contributing to them forgetting what they are supposed to be doing, but it can't be helping either.

Resetsim doesnt solve it; they go back to losing motives as soon as they show up again.

The register and bar sims wander around from time to time until another sim comes up to the object to interact with them. They are in "library" lot (an internet cafe with the lot set to library) so they like to go read a book or sometimes play on a computer when they dont have a customer -- which I dont mind so long as they get back to the register/bar when summoned; it makes the lot seem more realistic. But their motives are actually dropping, which is almost certainly pushing them to do all these things (including eat and use the bathroom). If no other sim approached them, they finish out the action, fill the need, and go back to business as usual. But if they are interrupted, then their needs continue to fall and possibly crash.

I have seen in multiple posts that role sims' needs are SUPPOSED to be locked by default so they can stay at their posts (in order to avoid this exact problem), but the special merchant sim (which is listed as a role) goes home at night and cannot be bought from, so dunno what is actually supposed to be happening.

You may recall I tried fixing this exact issue before with SP, but they ignored the locked motives in their castes, whether set by default or manual manipulation, and continued to drop as if they did not have the caste. We finally assumed the game was just processing too much for the NPCs AIs to keep up, due to the mega mall I had (which had 30+ sims on the lot 24/7). That obviously isnt the case here, since it was never part of this game. The other residents and NPCs on the lot are also displaying some "lag" in fulfilling their needs, but not to the point where they wet themselves or pass out -- yet.

I checked my demographics, and this is what I see:
Households 33
Residents 81
Service 51
Homeless 6
Tourists 2

Thats really not alot of total sims -- at least not enough where I think it should be causing the problem. My lots ARE active, but no more than a few sims per.

One thing that does stand out to me, though: 51 service sims. That seems like a huge amount for the number of households and residents. So I ran a demog on them and I only count 36 in the service sims list.

Which leads me to another oddity....a while back, right after one of my sims gave birth, I had a flood of butlers showing up to my house. I think my current (at the time) butler quit because her bed unassigned after a week of service (which is apparently a thing) at almost the exact same time. It seems somehow I got a weird version of that infinite birth glitch in the form of a butler - dunno, finally stopped the flood by terminating the service and it hasnt happened again. But I wonder how the game handled all those extra butlers it generated out of nothing, and if that is where those missing 15 service sims are? Spitballing here of course.

Anyway, I cant figure out how to find out what is causing the discrepancy between the service sims count. If there are 15 "orphaned" sims out there, it may be possible that is causing the game eat up processing resources...?

51 service sims is actually normal with all EP's installed it can get as high as 70-80 they don't actually use much in the way of resources since SP whether EA's or Nraas doesn't do anything progression with them you'd be surprised at how little they actually use now if you had a couple hundred or more that would be different
Demographics also counts Role sims as part of the service pool with the exception of tourist's
How many Role sims do you have in this world?

Demog Export, no filters:

Households: 33
Residents: 81
Service: 51
Homeless: 6
Tourists: 2


Demog Export, Service only:
Households: 0
Residents: 0
Service: 36
Homeless: 0
Tourists: 0

Demog export, Role only:

Households: 3
Residents: 0
Service: 0
Homeless: 6
Tourists: 0

Still leaves 9 unaccounted for. Not concerned about use of system resources on "normal" service sims, really. Worried about whether or not those missing sims are stuck in the code causing issues because the game doesnt know how to deal with them (like when sims get stuck in between walls or on elevators, etc and it causes the game to waste more and more resources on them until OW cleans them up).

Depends if it's time for their shift to start so not unusual to not see them around or show up in the Demog Export but 15 role objects for Sunset Valley is a little to much I would cut back the number of role objects.
I have at most 9 when playing SV

Unless this is Lynx's version of Sunset Valley which has much better routing and can handle more

I only have 6 role objects, which is why I only have 6 role sims. The only mention of "15" comes from the unfiltered Demog of 51 Service Sims minus the 36 Service Sims listed in the Demog, Service-filtered. Futher reduced by the 6 role sims, that leaves 9 service sims not showing up. Shouldnt they till show up in a Demographic report, even if they are off shift? (its 5PM game-time BTW, and even the maids are showing up in the Demog, filtered to service sims only).