Racks are odd things they kinda work on an average I always get variations top to bottom. I personally found the bottom the coolest and top the warmest but that is me. Is the flexwatt a single strip or a bunch of strips all wired together? A number of bits all together a poor connection especially if the bottom is where the leads go to the herpstat. What is the ambient air temps it sounds like they are low to me racks generally do best at 80ºF rooms.

I have never found a rack to hold better humidity than much else. Well ventilated tubs don't hold humidity better than an enclosure or tank if it did I would become concerned that there was not enough ventilation. My rack matches the room RH.