Hello, I'm surprised this issue wasn't mentioned elsewhere on the internet, but thankfully there's place I could bring this up.

Here's the problem, when I started the new town, Sims will park their cars at the available parking lot when they visited the community lot. However, after several Sim days, Sims, active or not, no longer park their cars at the community lot they're visiting. Instead, they put the cars in their personal inventory as if they're visiting the lot with no parking space (cars stop showing up in empty parking space).

This issue doesn't occur on residential lots and most career rabbit hole lots, but it tend to occur in other lots, like parks, library, hangout, bars, lounges etc.

I did try to reset the lot and objects with MasterController, but it doesn't resolve the problem. Although replacing the lot does. I disabled the Overwatch vehicle clean up and it seems to delay the issue a bit, but I don't think there's any causation between that. I never have any custom contents installed too.

This issue is resurfaced with every new game, across every towns.

Are there any explanations and possible solution to this issue?

I would just remove the parking spaces and leave it as a decorative parking lot.I don't allow any plarking spots except on a residence in any case because it leads to vehicles being deleted.

Nothing you can really do about that it seems to be just how the game is I have seen that in every world I play in as well
TadOlsen OW doesn't delete cars that are owned by inactive residents

^No, but it does tend to delete the free cheap cars that the game issues to inactives who don't have one or whose cars are currently not available to them, and who subsequently abandon the freebies. If the player is not paying attention, it might look to them like actually purchased cars are being deleted. I've been fooled by that myself more than once.

This is why I try to use the "Always Use Taxis for Inactives Lacking Vehicles" in Traffic, but of course in some worlds that just looks strange.

Well it might have something to do with the fact EA never bothered to release the cars from the parking spots... which means once one car is put in the spot, it's garbage. A bug that's apparently been here since day 1 and no one said anything.

Is that like when I move my sim's ex-girlfriend out put her car in her inventory before she moves so it's off my sim's lot but magically returns to his lot after I do a reset on the town ~

That sounds like something failed when you added it to the inventory actually.

It also happens when an inactive moves to a new house I noticed it once when my sim's neighbor that after I reset the town their cars reappeared back at their old house

I have been industriously deleting cars and parking spaces that come in downloaded lots, parking lots anywhere in town, decorative cars, etc, and replacing many of nthem with bicycles. Wishful thinking on my part. I wish I could give my sim worlds buses, or that the subways didn't work so badly. (in Bridgeport my sim ran several blocks out of his way, took a subway to the wrong part of the map, ran a few more blocks, took aother subway, and then ran to his destination, thus taking a couple of sim hours to go a couple of blocks. Admittedly, this was before I had stocked up on mods, but I've been wary of subways ever since).

Anyway, my point, and I do have one, is that cars in Sims 3 are obnoxious as they are in real life, but so is Sim transportation in general. The taxis are the least obnoxious element.