Bug in single-Single kerning exception?

I’ve been struggling to make a single-single kerning exception. I made a video to explain what’s happening. I really need to improve speech skill.
Basically the problem is that, when you make a single-single exception, the status of padlock (the group) seems to change elsewhere, though it shouldn’t. Also it can sometimes get it right in one master but not in the other(s).

I have had some problems along that same line. The workaround I’ve been doing is set any new Group to zero initially. It seems to help prevent problems later.

That’s what I do in the video too (also in my actual projects), and it works sometimes, but not always. There seems to be more specific trick you have to perform…