Quick Group panel in Keyword Manager window in Photos

I'm experiencing some strange behavior with the Quick Group panel in Keyword Manager window in Photos. I now have 22 items in that panels and want to drag one more up into that panel from the main Keywords panel. That works fine for all of my Keywords except for one which simply won't let me drop it up there. It lets me drag it but when I try to drop it, it just doesn't get deposited in that Quick Group panel. I can't see anything different about this one keyword. Any ideas?