The probe should be touching the flexwatt. It should not be in a bin. Tape the probe directly to the flexwatt.

Another thought is room temperature, what is the ambient temperature of the room? The colder the room the more heat generated by the heat tape. The localized heat rises at the back of the rack making the top boxes hotter.

Third, thought. I presume you assembled the rack, so you installed the heat tape. If the heat tape has a natural temperature gradient to the (current) top end, uninstall it, and put it back in upside down to its current position.