With "Scale Max Health" on true losing any form of health boost (Potion effect, items stats etc.) will deal damage varying on the level of health boost given and amount of additional health with the boost. It seems to be affect by the amount of additional health when having health boost. For example with health boost 2 will do no damage when at the default max but when at the health boost max it will deal a total of 3 hearts distributed across random body parts.

Losing a boost with additional health should just give you max health on all body parts