You need to follow the instructions for the brand & type UTH that you're using- they're not all the same. What I use (Flexwatt) says no use of foil or on metal shelves (presumably because there isn't much insulation over the electrical pathway & they don't want anyone or anything to get a shock). Also, mine (Flexwatt) is not self-stick so I hold it in place with a little tape at the edges, but not over the embedded wires.
I don't use the kind of UTH that's made to be self-adhesive, but as far as I know, all UTH is supposed to have a small ventilation gap for safety, sofor doing that & also for having yours connected to a t-stat. I've seen other posts here that some use foil tape to fasten their UTH, but I can't say for sure about how you've covered the UTH to hold it on- it's not that foil tape is insulating though (it shouldn't make the UTH over-heat) & it conducts heat very well. (It's just that foil tape conducts electricity rather well, so if your UTH ever got damaged or cracked it could be a problem.)
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