I think it's clear that your probe is getting too hot too fast. It's reaching the set thermostat temp, so that's why it's not getting any hotter in your tubs.
Do you have a temp gun? Can you measure the exact temp of the flexwatt and probe?
Maybe you're some how trapping the heat under your tub, so that's why you're not getting an accurate reading?? Or you can just set your thermostat to a higher temp and accommodate the tub temp
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