Dissapearing sims??

This is an old problem, it has a bug report already https://answers.ea.com/t5/Bug-Reports/OPEN-NPC-s-not-interacting-on-community-lots-despawning-too-fast/td-p/8195638 you can click “me too” on this report if you want.
The fireplace exit is a bug that came after seasons, as it’s how father winter arrives and leaves on winterfest.

So, a couple things.

Walkbys (any sims):
If they're walkbys and they happen to already be leaving at the time you try to start a conversation, you usually can't interrupt them leaving.
If they aren't about to leave, you can chat briefly. But they do still have a walkby timer running, so at some point they will leave and you won't be able to stop them.
But, if you use "Ask to Hang Out," and they say "Sure I can hang out for a bit," then the timer changes from the walkby timer to the longer timer used for friendly visits, several sim-hours.

Celebrities: (if you meant sims who have a celebrity rank in the game)
You can't interact with celebrities beyond the "Attempt introduction," "Ask for selfie" and the couple other interactions at first. If Attempt Introduction fails, the conversation will end and the celebrity will probably walk away. You have to wait till next time you see that celebrity sim and try again.

@mightysprite ok thanks for the info, and by celebrity I mean a person on gallery created a sim that looks like a real life person/celebrity....like Emilia Clarke for example..

@Kita5399 Ok, i didnt check, I guess I should...thanks for the info.