The bug

When you get the health boost effect, it will always set your current health to 20 HP even if you have more or less. This is most noticeable when using a repeating command block.

How to reproduce:

1. Place a repeating command block
2. Put this command in it: effect @p health_boost 9 9
3. Activate the command block
4. Go into survival mode
5. Take damage (For example fall damage) or heal yourself using a healing potion.

Observed result: Your health will always be reset to 20 HP, which is full health when not having the effect.
Expected Result: Your current health shouldn't be affected by the effect, so you should normally take fall damage without instantly being healed.

Original Description
Using a command block: repeat, always active, effect @a health_boost 20 4..
You have the Health_boost effect, but you can not regen any of the 2nd healthbar. The command keeps giving health_boost, which is an empty health bar, removing any health gained instantly. Boosts are positive, they should not remove health gained. It also worked fine in the previous version.

My personal use for this command is to allow players of my custom map to get permanent health_boost, as I am understanding, this is the only command and it differs in outcome of the previous game version.