I'm 99% sure I "solved" the problem - or at least remembered the cause, that seems to be it ... It seems that the Key Bindings don't work if File Explorer is open.

I know that sounds 100% crazy, but I found a note to myself that said that. I closed File Explorer, loaded my most recent version of my add-in, and voila, they are working now.

Go figure. So just a waisted day until I remembered/re-discovered this Windows bug.