Yeah there is no way heat tape regulated like that should be scorching like that. Only thing I could even think of is too long of a strip for one thermostat to adequately regulate but then again people do this all the time without it doing this. If this is a 4 shelf rack that would be even worse. Something is definitely up here.

Wait did you have this controlled by a ve 100 or a herpstat? Ve 100 is on off control and not proportional so just like having a herpstat it can run far hotter than the set temp. That would be my only other thought. You could have it set to 90 on say the bottom rack and the top gets to 100 due to using an on/off controller like the ve-100. Again that is wild speculation because I am sure others do this as well. If a portion of that tape got over 110 degrees, given enough time and having a tub over it could insulate it to over that and it could scorch. That is just theoretical minimum to cause a thermal burn but again this shouldn't be happening.

Not knocking RBIs thermostats but anything less than the VE 200 is not proportional heating.