Can't add Accessories to a Homekit Scene

Hi,

I've previously set up Scenes on my iPad and iPhone. Now I'd like to edit a few (added a few new Hue Bulbs that I'd like to add to the "Good Night" scene. I can get to the edit & add/delete accessories screen on all of the devices, but whenever I click on an item to add, it shows the check mark, then the check mark disappears. This even happens on my Mac, running Big Sur 11.01. Anyone else have this problem? It won't even set up a new Scene anymore.

I wonder if I'm missing some setting that allows me to edit them, or if a third party app might be mucking it up.

Would love some suggestions.