BTW, I just tested on my Mac with the most recent system version, macOS 12 Monterey, the bug is still persisting. The only themes, where I can play a video full length in a slideshow, are Classic and Magazine.
Live Photos can still not be played as an animation in a slideshow. my impression is, that Apple is now giving the Memories more attention - they seem to be the successor to slideshows.
I am creating my slideshows with Keynote, where it is much easier to add text to the individual slides and I have more controller the sound effects.