It's plenty easy to get the healing potion. Just enter turn based mode and jump over. Stalks adhere to the TB rules so they won't explode during TB mode until you end your turn. (This is also useful for when saving Baelen and/or collecting the Noblestalk. So long as you don't end your turn within their detonation range, they won't do anything)
But it's as easy as jumping over, enter TB mode, grab the potion and the chest, then jump back to the ledge and turn off TB mode (I usually then drop the chest at the feet of my party and then disarm the trap and pick the lock then)
No need to ever take damage. Unless you disarm the trap by having Karlach detonate it because she only takes 1-2 damage (If she doesn't get the Dex save to avoid it completely) because traps in this game are hilariously terrible and it's often faster than messing around with rolling the D1 the game uses (For me anyway)
I've never seen the effects of the potion occur when actually grabbing the bottle though. So that it occurred for you sounds like a bug.
