Hang on... just realised something:
Right after the crash with Roglic the mechanic started running towards the front straight afterwards (from way back in the car-line)
Roglic finished on Vingegaard's bike.
Did the mechanic arrive at the crash-scene, only to be told that "Oh, he's got Jonas' bike."?
Did Vingegaard then get Roglic's bike, and finish on that?
Did they never, at any point during the remaining 40+ Ks of the stage - and I do recall it calming down a bit - look at each other and just... you know... swap back?
Right after the crash there is very little information, and in that situation you don't wait but react (obv Roglic is main priority). When things calmed down and peloton was back together, it just didn't make sense to swap bikes again and risk something. Jonas and Roglic are similary build, so bike was very close to what he is used to.