[GAMEPLAY] Possible MR / damage taken bug with Rift Herald?

I'm not sure if this falls into "spec mode" bug or actual "dealt too much damage" bug.

I record most of my games via spec mode using Baron Replays. I was watching through the results of the above linked game especially because of the Rift Herald steal that occurred at 15:15. Lee Sin smited the Herald at 15:14 to leave it with 141 HP, and then Nami's (my) tidal wave hit it at 15:15 and stole it. Even on 0.25x speed, the spectator replay does not show any other damage occurring between the two events.

At the time, I had rank 1 tidal wave (Level 8) and 27 Ability Power. The tooltip for Tidal Wave shows 150 (+16) damage, and Rift Herald's tool tip shows he has 20 MR. I have no items, runes, or masteries providing me with Magic Penetration. By my math, 20 MR = 16.6666% damage reduction, which should mean I do 138 (and some decimal place) damage to Rift Herald.

So, I'm not really sure how I stole it. One thing pointed out to me by a buddy was the Tooth / Nail passive might have proc'd to steal health right before the Tidal Wave damage applied, but you definitely cannot see that from my spec mode replay. Also, the replay does not show any health gain for Lee Sin after the Smite.

I can upload the Baron Replays file or clip the pertinent video out of it if necessary.