Well surprise, surprise, qt 6.8 is still failing with:
Assertion failed: _UM_CRUMB_STATUS (crumb) == _UMS_FREE, file ./libc-0.1.13/src/emx/src/lib/malloc/ifree.c, line 30
TRP attached. I should probably do a debug build to make the TRP more useful. I'll have to dust my notes off from when I was last conversing with Steven around this TRP.