Darn, haven't actually messed with magnet adhesives. Might it be 2 sided tape?
I would think it would not be something with a low forming temperature because you would indeed have trouble with motor heat, but it DID come loose so maybe it does become soft before the temperature of water boiling. Keep in mind that anything that removes it from one side of the casing will loosen the other battery.
Is what's left sticky, coming off? Acetone or citric acid might take it off. I can't see it, no idea how important it might be to remove what's left. I would be concerned that you don't create a thicker layer than was already there and push the magnet closer to the armature.