Sorry it took so long to get back to you on this. I've seen people with this issue on Reddit, so I decided to figure it out. Had to start a new file and then do some looking.
I can confirm that Titans do, in fact, respawn. They use the same in game flag as Legendaries and (surprisingly) 7 star Tera Raids to check if you've caught them yet.
Here's what I did exactly using Pokémon Violet.
1) I knocked out Iron Treads.
2) I moved the time on my Switch forward by 1 hour with the game still running.
3) I saved and closed the game.
4) I opened the game again.
5) I hydrated (always remember to take care of yourself).
6) Iron Treads returned
So some things to note that didn't work.
Completing the Ace Tournament didn't work
Resetting immediately didn't work
Moving the time forward without resetting didn't work
This also worked on all 5 legendaries, so I'm guessing GF put a failsafe on special static encounters.
When I checked on Reddit, a lot of people just don't seem to close out of their game, so I'm guessing that's what happened with you?
Either way, good luck!
