I have no clue what the problem you're describing could be there, but this reminded me of very old DMUs that did have such a problem: they needed air pressure to engage or disengage final drives (in forward or reverse), and they couldn't be towed with them engaged (because that would damage the gearbox that was receiving no oil flow). See https://www.youtube.com/watch?v=XCQKvEy6jVs for a description of that ancient train's failure modes.