What fails: If I open an existing Media Player with one MP3, remove that MP3, then add a different MP3 and close that Media Player, the file does not play. (I have encountered other variations of the failure mode, but the most frequent is “it does not play”)
What is OK: If I open an existing Media Player with one MP3, remove that MP3, close the Media Player and reopen it, then add a different MP3 and close that Media Player, the file does play.
as far as I know that’s not a bug. The file you added after you removed the other is there in the playlist but it is not in the default slot anymore. So when you replace the file that way you need to open the hamburger menu on the media player and select the new song you selected.
use hamburger menu to load the track, notice that (None) replaced your original file’s slot and the new file you added is directly below it. Selecting it will move it to the top position.
Another way that loads the file immediately into the top slot is to drag and drop the file directly onto the track field. In this case it recognizes that there is a file already there so it assigns a track number to the one you dragged over and moves it to the top of the playlist in one go.