Sounds to me like your problem may be the rheostat. You should have it 1) thermostat plugged into the wall 2) rheostat plugged into thermostat 3) UTH plugged into rheostat.
I would remove the rheostat and just use the thermostat and UTH to see if the problem goes away. None of my tubs have ever melted or burned up a UTH, and my heaters are coverd with a piece of polystyrene insulation. I think this may have been happening in your glass tanks if you were using the same set-up, it's just now you are seeing it because of the melting, which is why I think it is electrical...
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