At some point today I went and updated Elvui and whatever else was in the queue on CurseForge (sorry, I didn’t pay attention which ones), but after that my shift modifier stopped working.
The only way I can get it to work now is Esc ==> Options ==> Action Bars then set the “Lock Action Bars” button to “None”
The default is Shift.
Surprise, surprise - if I change it to Alt for example, then my Alt-modifiers do not work.
Hope this helps someone.