The zener diode at VR1 is partially in parallel with R56 so may affect its reading in one direction.

Checking two boards here, I got the same reading in both directions across R56 with one board and the other board there is about a 1.5k ohm difference in directional readings.

Lift one leg of VR1 and measure the resistance across it in both directions. See if it's what's causing the resistance variation.

With R51 and R55 blown, I presume you replaced the three transistors? They usually all die at the same time.