Bard bug ultimate

During an aram game, we were taking down the enemy tower. right when the inhibitor was re-spawning, bard ulti's the tower then the inhibitor re-spawns. the tower became invulnerable for the time being, but when the tower was out of the bard ulti, the tower was attackable and the nexus while their inhibitor was up. I tried to do this myself in a custom game and the same thing happens. i was able to attack the tower while the inhibitor was up.

inhibitor up towers attackable while inhibitor is up (see map)