Here are the screenshots BeckyEbook suggested, or as close as I can get to them, in case they will be helpful. They are taken on two machines, both running Kubuntu 23.20, one with Sigil 2.1.0 and Qt 6.4.2, and the other with Sigil 1.9.30 and Qt 5.15.10.
The exact same epub file is used in both.
As you can see, the one with the older Sigil and Qt (on the right) displays bold as it should, while the newer one (on the left)does not. The active CSS is the same.
I will pursue the font-engine issue when I get the chance. For now, life and the cats are calling
Thanks for all the advice so far!
Albert