I'm gonna have to assume it has to do with Milan forming thr ambrosian republic, but that's not supposed to happen while they're in a PU
Correct, it's a same-day decision. I've seen this before.
Immediately after their leader dies without an heir or queen they fall into a regency and check the decision to turn into a Republic.
However, if they're in the tier where they'd get PU'd they also fall under a PU, which they can't since they're a Republic.
If it gets contested their new "overlord" has to fight a succession war without getting the PU.
Incredibly rare, but it happens just like OP described it.
Dunno if it's new (don't think so) and can't remember if I reported it before, but it's a bug.
EDIT: Could be fixed by delaying the decision by one day. Only that specific decision though. Other decisions should stay instant for various reasons.