What does "invalid mode 'kCFRunLoopCommonModes' ..."mean?

Judging from the replies by Apple Developer Relations to this post, this is a bug in UIKit and one that Apple is tracking. Until the bug is fixed, the advice is to treat this as "log noise".

I think it's a warning that Apple should fix it itself. Even in this sample project which is from WWDC19 this problem exists. There is a UISwitch in a cell of a table. When I tap it, the mentioned warning is raised.

So, in my view, it is a bug that Apple should deal with.