Thunderbird MOVE TO and DELETE buttons are disabled when moving a message

For some months now a recent update of Thunderbird has messed up a key functionality for me. When I open a message (which in my settings opens a new tab with the message), and I use the MOVE TO dropdown to move the message to some other folder, it will load the next message in the list. So far, so good. However, after moving the message, about 30% of the time the MOVE TO and DELETE buttons are faded out for that next message that appears. I don't understand why - or how to fix this. Any thoughts? How it looks BEFORE moving a message: And this is what happens with the next message: