Inhibitor/Nexus Turrets reducing true damage

Pretty much what the title says.

The beam of Inhib/Nexus turrets even reduces true damage, but by my understanding of Riots policy on true damage (no increase/decrease by ANY means) this shouldn't be happening.

Observation: Attacking a Inhib/Nexus turret with elixir of sorcery active while having aggro (a.k.a. getting the debuffs of the turret) will deal 21 true damage. Would perfectly fit in with 15% damage reduction (which shouldn't be happening?)(25*0,85=21,25, rounded to 21). Forgot to test this on champs.

Tested: On Inhib Turret (Mid Lane) and both Nexus Turrets. With Ahri. Perfect reproducibility (around 30 times). Attacking while not being debuff'd will do 25 true damage as intended. Also forgot to test this on champs.

Screenshot: Sorry, forgot one [Brain OP]. Should be clear though. Instead of a white 25, which is meant to pop up next to the red number, it is a white 21.

(Cross-posted from euw formus. Dont blame me)