BLU Basic Instinct heal buff does not apply to the regen effect from Spick-and-span

[XiAshtra]

Isn’t this a snapshotting issue? Healing spells and abilities don’t benefit from buffs that were used after the fact. I.E if you were to use Medica 2, then Asylum or Temperance, the regen effect wouldn’t be adjusted because the buffs were used after the regen was applied. The same is true of damage buffs and DoTs, they can only benefit from buffs that were present at the time of application

Does the same still occur if the Basic Instinct buff is active before the regen from Deep Clean is applied?

No, it is not. You didn't follow the steps at all. I am letting the first Spick-and-span run out completely, applying Basic Instinct (which has infinite duration from the moment you apply it), and applying an all new instance of Spick-and-span. Try it yourself and you'll see the issue.