Seems like a similar issue I had with one Clip+ I had. I only had this MP3 player for a few month and I had a 90 degree piece in there so I didn’t have to push in and out using the jack. It does stick out a little this piece.
And this issue is clearly the MP3 player as I tried the headphones on a different mp3 player.
I do wonder if you had pushed the headphone plug in all the way. The player’s headphone jack fit for the plug can be very tight at first, but then loosens up after a time or 2.
That certainly isn’t the case as it is in the whole way as if I had for 3 month and started have this problem.
I would call SanDisk Tech support about a warranty replacement then.