What I expected to happen was that the effect would increase the total health of the player for the duration of the effect.

What actually happened was that the player periodically took damage from an unknown source equal to the health gained from the effect.

Steps to Reproduce:
1. Open a creative world with cheats enabled.
2. Change gamemode to survival
3. Use the following command, /effect (playername) 21 100 1
Steve? will gain 4 extra hearts, but will also periodically take 4 hearts of damage.