The bug(s):

When a player loses health, giving them health boost would bring them back up to 20 hp. Similarly, a player would regain lost hp (if they lost health) when the effect ends.

How to reproduce: (1)

1. damage yourself using any methods
2. effect yourself with any level of health boost for a few seconds

Expected result: The player's health should not change when health boost is applied
Observed result: The player's hp gets set back to 20
 

How to reproduce: (2)

1. effect yourself with any level of health boost for a few seconds
2. damage yourself using any methods
3. wait for the health boost effect to end

Expected result: The player's health should not change when the health boost effect expires except for any extra hearts that must be removed
Observed result: The player's hp gets set back to 20

These glitches are more of an addition to the known health boost issues, but I had to address this to ensure this doesn't get swept under the radar.
I have not tested this with the 1.19.1 preview so if this is fixed in 1.19.1 let me know.