First of all let me say I have no idea how the gas and electric hot water systems work in the TM. Now, having established my ignorance I can ask a silly question.
Are their separate thermostats on / in the tank for the gas and electric heaters? Can they be cross wired in the manufacturing process? I have no idea. But since you stated that you only recently started using the electric part of the tank my curiosity would take me there first. And hopefully not do to me the same that it does for the cat....
Just thinking out loud, sort of.....typing out loud... kinda....
you get the idea...