I'm having a similar issue. I've been hammering away at it for a while. My LibreELEC card works normally on a Pi 4, but on a 400, I can't even boot properly (circumvented by booting without a card and inserting it part way through). My present conclusion is that the 4 and 400 aren't 1:1 and the Pi 400 isn't properly supported at this time.
Edit: I found this post. It explains exactly why the 400 doesn't work.