Had a free moment:
See: 
https://github.com/kevinhendricks/Bu...or_sigil_1.0.0
Download:
Sigil.app-1.9.0-Beta-Qt6-Mac.txz
Caveats:
 - This build is totally unsigned as my code signing routines are not set up for Qt6 yet.
 - The Qt6 port of Sigil is very experimental.
 - Although this is master it has lots of internal changes that make it  *Beta* and so should not be used in production without lots of prior testing
 - Because it is Qt6, it will *not* work with PyQt5 plugins (only PySide6 plugins)
    but it will work with tcl/tk plugins just fine.  Tweaking a PyQt5 plugin to use 
    PySide6 can be done reasonably easily.  We are working on a way to make this even
    easier and something that will support both in the future.
I posted this just to test if Qt 6.2.3 has a fix for your bug in Monterrey.  
If so, I will try to track down that fix and backport it to Qt 5.12.9 as a MacOS specific patch hopefully in time for the upcoming release in about 2 weeks.
So please let me know if it helps in that regard.