I've reported it to vue-mode, but vue-mode is a thin wrapper around mmm-mode (mode for multiple major modes in the same buffer). It may be hard for the mmm-mode maintainer to understand my issue since I'm using a derived mode, not mmm-mode directly.
I was hoping Stefan Monnier might chime in with some ideas -- I think I'll have to dive into it, and I'm OK with that, but could use a bit of guidance. I suspect it's related to the fact that mmm-mode is switching major modes within the same buffer and trying to adjust everything on the fly, even when the buffer hasn't changed. (In mmm-mode it switches modes when you enter a certain region of the file.)
For instance is there any way to temporarily completely turn off the syntax-ppss caching mechanism, so I can really see if that's the root cause?
I can definitely confirm that holding everything else constant except that single emacs commit of Stefan's, 0b3982b1a38, triggers the failure. But I understand it's probably some interaction between that and mmm-mode.