On two seperate themostats?

I have three reptitemp 500r's... one is controlling just the heat panel in the melamine box for my Afrock

The other one is controlling (1) 24 watt zoomed UTH on the bottom of a sterilite tub, and (1) 15 watt UTH stuck to the back of the tub.

The other tub is using an entirely sepereate 500r, with the exact same setup.

In both tubs, the Zoomed UTH has failed twice, each one independent of the other on seperate 500r's. The other UTH on the back of both of them, have not failed.

It could be they just warp too much and flex out on the sterilite, and causes the fine wire inside the pad to snap and melt.