I’ve been able to reproduce this behaviour on multiple computer with the current Firefox. Edge doesn’t 100% reproduce this exact problem, but there’s also similar weird stuff going on with scrolling.
Just try it on my blog, podcast.vielgeliebt.net - open the webpage on one monitor, close the transcript (shown by default) and then while it is playing, move it over to a second monitor. The player will somehow “scroll away”, it looks as if the Wordpress footer would actually scroll on top of the player. The podcast is still playing in the background, but can’t be controlled. Only refreshing the page brings the player back.
When the transcript (or any other of the tabs/menus) is opened, the problem doesn’t happen.