@simontatham I was going to suggest the built-in xkbmap option to turn LShift+RShift into Caps for such situations (as I usually run with -option compose:caps, it goes well with -option shift:both_capslock)...

Something else, vaguely similar, used to happen for me when I was doing nested RDP connections from Linux – FreeRDP into a VM host, then console into a VM – the X server would sometimes get stuck thinking a nonexistent /Shift/ key was pressed, just from watching a VM boot. What really made it weird is that it wasn't in Caps mode, as even holding physical Shift wouldn't return to lowercase (as normally would with Caps + Shift); it was as if I had a third Shift key stuck.

I could never figure out any any way to undo that, short of restarting the entire X server. Perhaps fortunately, Xwayland seems to be more resilient to that (and less impactful to restart anyway).