I don’t have any specific details on this, I didn’t have time when I last saw it to try scanning through the developer console.
Sometimes the Play/Pause and Stop buttons at the top become disabled and grayed out (circled in image below). I think they also become disabled in the dropdown menu, but I don’t remember. I’ll update this when I next see it and can confirm either way. My guess is that this happens with long-lived browser sessions.
I’m unsure if this is local to our environment, or affects others, so I thought it best to ask here if others have seen that.