What I expected to happen:
When given the Health Boost effect, the extra hearts will regenerate.

What actually happened:
When giving the Health Boost effect the extra hearts do appear and are 'empty' like they always have been. However, these extra hearts don't regenerate naturally. They neither regen when given the instant health effect (no notice-able change at all) or with the regenerate effect (the extra hearts behave glitchy and you can notice they try to regenerate, but fail to do so).

Steps to reproduce:

• Give yourself a Health Boost effect: /effect @p 21 10 2 true or /effect @p minecraft:health_boost 10 2 true
• Wait with full hunger and you will notice they don't regenerate naturally (while the normal standard hearts do)
• Give yourself a instant health/regenerate effect and you will see the hearts still don't regenerate (while the normal standard hearts do): /effect @p 6 10 5 and /effect @p 10 10 5 true or /effect @p minecraft:instant_health 10 5 true and /effect @p minecraft:regeneration 10 5 true