Janna Shield Saves Turret

I played a game with a support. We were getting rekt and the enemy team was pushing the 2nd turret in *mid lane *on the blue side of the map. The turret was about to be destroyed, but when shielded it, it went back to FULL HEALTH INSTANTLY. Even with the passive shield reset back on it. It only happened to me once, but I figured I should let Riot know of it actually existing. I also don't know if it's just that turret on the Blue Side of the map. I think the turret was either just taking lethal damage as the shield was placed on it or close to no health.