I also have this issue, My allies can still do it, and renouncing the ascendancy doesn't fix the issue either. Hoping this gets noticed and fixed soon.

I have a similar concern. After winning a war as an Ascended Empire against another Ascended Empire, the looser gets banished. The loser can secretly re-arm through edicts and come back from being Banished (puppeted) to being Ascended again. The winner has a choice of going to war against the looser again, or just leaving the looser alone through an event. Going to war is very problematic, as all of your Federation allies align against you with the loser (for some crazy reason, mostly related to bad coding I think), and it's only a war of annihilation (which I don't want as a goal).

Now if you choose to leave the looser alone, it can't regain full independence, even if it ascended again. There's no diplomacy action for it at all, and console commands don't work. The looser (who ascended again) just has no choice but to be stuck with the banished empire status and can't leave the Federation, or do much of anything really. I don't think this is by design, but if it is, I very much dislike this, either way. Re-Ascended Empires should have full independence.



Edit: I ended up just editing the save file found here: Documents\Paradox Interactive\Stellaris\save games\... (opened the .sav file with a file compression programme like 7zip and used notepad++ to open up the gamestate file). I removed this entry, after searching for the term "banished":

overlord=0
subject_type="banished_ascendancy"
subject_date="2402.12.15"
starting_system=222
has_advisor=no
shown_message_types={
"DEFEAT"
"WAR_ENDED"
}

It worked, but it's obviously not practical.