BLF188XR Smoked

The BLF188XR in my amplifier was recently replaced (think a voltage spike took out the original).
The "Arctic Silver 5" thermal compound was used for the mounting. According to the specification, it is non-conductive, although it may not be non-conductive enough. The chemical could be to blame for the following issue.
When I connected 48 volts to the drain, I witnessed a tiny spark and a small cloud of smoke (s). I couldn't track out the specific source.

After that, I delivered 400 watts to a dummy load using a 14 MHz RF drive. I almost immediately got arcs, smoke, and even a little fire at the drain tabs! Before I could switch off the power, one of the drain tabs was shattered, and the other was burned to the package casing. A lead tab is no longer available. The char/carbon from the burned adjacent circuit board was scraped and ground away.
There will be a new BLF188XR on the way. Plan on changing the next one (pain!) with an alumina-based thermal compound and hope for a better outcome.
Is there anyone out there that has dealt with a similar situation before?