BLF188XR Smoked, why? Thermal compound?

I just replaced the BLF188XR in my amplifier (think a voltage spike took out the original).
“Arctic Silver 5” thermal compound was used in mounting. The specification indicates it is non-conductive, but maybe not non-conductive enough. Suspect the compound was responsible for the following issue.
When I applied 48 volts to the drain(s), saw a tiny flash and a small puff of smoke. Could not find the exact source.
Then I applied a 14 MHz RF drive to get 400 watts out to dummy load. Almost immediately, got arcs, smoke, and even a little fire at the drain tabs! Before I could kill the power, one drain tab was damaged, the other tab burned to the package case. No lead tab left. Was able to scrap and grind away the burned adjacent circuit board char/carbon.
A new BLF188XR is now on order. Plan to install the next replacement (ouch!) using old school alumina-based thermal compound to install it and hope for a better outcome.
Does anyone have any experience with a situation like this? Any comments are welcome.

Looks like you are talking amateur radio (20 metre band :slight_smile: ) on a HiFi forum?