This seems like an electronics issue. We’ve had two motors fail, both in what seems to be the same way. The brush/commutator bit appeared to melt. The thing we noticed with ours is the sound from the motor. When we fully disassembled the casing, and removed the motor from the board (it snaps in) it made rattling noises. We decided to then take it apart (I don’t recommend if you want a replacement), and there was a solder blob, along with (what I assume to be) the commutator (which fell off when barely touched after removing the armature) with some blobs of solder on it, like it melted during operation. Here are some microscope pictures of the motor:
I am by no means a expert on Brushed DC motors, so I may be interpreting this wrong, but in my case it looked like an overtemperature issue. This was because it has a physical limit and was trying to reach a value it never could. It never burned out, just continued going until its death.
Were you trying to make the motor rotate for a certain distance, or using constant power?