I stumbled on this problem with conditional_fields, but the issue is in the core state.js. I just have 1 Select-Field, which allows multiple-values, and wanted to have a text-field to show up when an option is selected.
So the text-field should be hidden initially, since nothing is selected, not even "_none" (which is another issue I'm not going into here).
But the text-field is shown until I select something. If I select the one option which should show it, it stays there, otherwise it's gone. So that works fine, only the initial state is broken.
I had the issue with D7, but it holds true for D8 and D9. I tested it with D7, it worked there... so I assume it will work with D8 and D9.