@FunkOff I suppose, that my issue with factory's build queue being cancelled also relates to this.
I haven't traced prerequisites of this issue intentionally, but it seems like indeed when you fast-select(through Shift) all units in the area around the factory and tell them to assist it, it's BQ just cancels itself.
Don't know whether this was before because as a 1100 noob, I've just recently started to tryhard with factory assists. Meaning that back in the day I selected engineers around the fac explicitly(all t1, then all t2 and then t3) .