Not looking for an argument, just a suggestion. If it did fail due to being on the 2 post ramp, there's a clue in there what problem(s) the car might have had before it went on the ramp.
Do you find it on bump stops over night? I assume not as you said it was working fine before. So if it's not normally leaking, the only reason for the bags to fully deflate on the ramp is if they had damage that is only able to leak when fully extended. If this is the case, your bags were on borrowed time.
I wouldn't expect an MOT to pick this sort of thing up. They just check if things work at the time of the MOT, not if lifting it int he air could disturb something.