Disclaimer: Before you mark it as non-reproducible, I wanted to mention that it does not happen every time and not with every character. I cannot quite put my finger on the timing or a particular scenario but I can describe what happens.
Suddenly when you start Face Puppet, all its buttons and controls become disabled (see screenshot). It does not freeze the app and it remains responsive. Reloading the project doesn't help, but restarting the application does. I don't know if it's related, but Motion Live 2D also becomes disabled sometimes. It happened to me on the same project and in the same session.
Again, it is not critical and you don't need to do anything to help me resolve it. Clearly there is some scenario where it occurs, so keep it in mind, and if you get enough feedback from other users, you might decide to look into it.