Putting an engineer on board the final flight means .... that they may have been accustomed to fixing / testing serious issues on passenger full planes, and his presence may have complicated the situation by either the pilots trying to activate the fault in order that the engineer can see the problem, or that the engineer used any arising faults as an opportunity to do some diagnosis rather than let the pilots recover.
Well it's 100% insane. Once you "put the engineer on board", your intentions are clear.