Calculate the capacitance of the transistor tab to heatsink with a .002 think mica insulator.
Do that first. For both.
Now ground the heatsink to the board ground. Not an issue.
Don't forget the RD16HHF uses the same package so it has about 4nh of lead and
bond wire inductance. That means the tab and the enad connection point at NOT
at the same RF potential. Ground the tab and the heatsink is now carrying RF current.
The RF coupling is already there via the output transformer.