[Imp]

I would say the designer did the following to cover his bets.

Had a few units on or by bridge with demo charges to represent the engineers wiring the bridge. I defenitly had a demo charge go off on the bridge but probably attacking a unit not the bridge its self on reflection but gives right atmosphere.

The ammo is indeed Hungarian so it belongs to the player & represents the charges placed by the Yugos. It arrives as a reinforcement about turn 14 could check in your HQ menu if wanted. The moment it arrives they start firing at it to blow the bridge when it goes bang as its an enemy unit.

It could now be replaced by the new explosive class so belongs to Yugos & would probably blow when a unit tries to cross. No need to fudge it anymore & you would have to disarm it before crossing. That would be most tricky as the infantry will see your engineer move adj & even if does not go off will probably blow it. Place smoke & you might get away with it but a human would probably press F.
For this Scenerio think it would make crossing slightly harder as cant move adj till you have cleared all enemy units from LOS so no ones left at which time the AI may just decide to make it go bang anyway.