I just had a very similar freeze with an mp4 on my desktop. sw or gl rendering. Preview pane shows a generic film reel picture sometimes, and if that happens then the compositor deadlocks as soon as I dismiss the preview. bt shows it's deadlocked in some gst mutex stuff, literally 193 stack frames deep.
I've just noticed I can't reproduce this at all without xwayland loaded, or if I do have it loaded but launch an xterm before trying to deadlock the compositor - so I think this may be in some way related to xwayland.
There's a message on stderr about glamor starting (or failing to in sw mode) around when the broken preview pane pops up.