I gave a thought to this heatsink problem, and the only reason i see for them to be live is EMI noise reduction (unless they're actually connected to the rest of the circuit).
Put 2 opposing sinewave signals in 2 opposing blocks of metal - in the middle you've got a flat line.
In this case maybe the sinks are live to compensate the unavoidable mains noise you've got on mosfets output.
The only way to work this out is with a good oscilloscope.
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