Regen does not regen extra hearts added by Health Boost. Also, if regen level is over 2 after you give yourself level 255 regen, you will take damage.

Steps to reproduce 1st bug:
1)Give yourself health boost (/effect @p 21)
2)Give yourself regen (/effect @p 10)
3)Note that the regen is not regening the extra hearts.

Steps to reproduce 2nd bug:
1)Give yourself regen of 5 (/effect @p 10 30 5)
2)Note that you are not taking damage.
3)Give yourself regen of 255(/effect @p 10 30 255
4)Note that you are now taking damage, even in creative mode.
5)Give yourself regen of 5 (/effect @p 10 30 5)
6)Note that now you are getting damage even though you have the same effect as step one.