It's happening since 2018 or 2019 version, I don't remember but for quite a while. The only response I got from unity team was "try a different layout" as if that is a magic solution no that untraceable bug. It didn't help. I've had a little different layouts, but generally it's like this, and I have to often restart unity because it's the only way to get inspector working properly again. So the issue: the inspector often behaves as if it is locked but it is not, and nothing can get it out of that state.