Yeah, so I think it’d be fine, since I’d think the charge limit would be about the same as the discharge limit of the power bank. It would heat up a normal amount for charging and being charged at the same time, but I don’t think it would melt down or anything. It’d just drain slowly over time.
I used to take those 9-volt battery connectors, wire them up together and then recharge a dead 9-volt with a brand new one until the tester strip thing showed they were both even. Surprised they never popped because they would get really hot lol