conpty hangs in _ClosePseudoConsoleMembers about terminal HOT 1 CLOSED

The pipes passed by Vim are all closed by the time it calls ClosePseudoConsole, I've read the remark in the docs regarding the possibility for the function to hang, but here I'm not sure it applies to this case.

It does apply unfortunately. You must keep the output pipe open and read from it until after ClosePseudoConsole has returned.

Because this was difficult to use, we have modified ClosePseudoConsole in Windows 11 24H2 to never block. But that doesn't fix the issue on older Windows versions of course. I'm afraid this is an issue that vim must fix on their side for now (especially since this flaw was publicly documented for a long time).
Windows 11 24H2 also contains a ReleasePseudoConsole API with which you can simply read the output pipe until it's closed on you. That way you don't need to monitor whether the spawned child has exited, which makes it even safer to use.

from terminal.