Engines instantly drain LiquidFuel when tanks unlocked

[leifgiering]

It's happening on multiple engines from this mod, at least the ones I tested (MATTOCK, ESTOC, and Aerospike, and presumably others). If you lock a tank in the SPH/VAB, when you unlock the tank in-flight the engines will instantly drain the tank completely. I'm using 1.8.3.4, testing it (along with dependencies) on an otherwise stock install of KSP 1.5.