When setting a player's base generic.max_health attribute (via /attribute), after the player dies and respawns they will be at 20HP with the remainder of the max health missing. It also does not allow healing past 20HP.
MC-179984
Respawning after modifying base max_health attribute leaves HP at 20
Resolved
Duplicate
1
Dallin Smith
(Unassigned)
Unconfirmed
(Unassigned)
20w17a
Retrieved
Happened to me too. You have to change the health again in order for it to work, but the hearts are still visible like you showed.