It the signals are strong enough to fry Q90 the mixer will soon be gone too.
The BE junction is as robust as the mixer diodes. so whatever you are seeing
is quite strong and could even be ESD.
For ubitx (V3 or 4) a pair of back to back 1n194/4148 diodes from K3 Pin14 to ground
is the best bet as then its only there for RX and has no impact on TX.
Bitx 20 and 40 the solution is different.
Its easily done on the back of the board.