The command I've used is /attribute @p minecraft:generic.max_health base set <value>

This command does update the max_health attribute but it does not update the player's health such that it matches the specified max. In other words, I'm able to have a health of 20 while having a max health of 5. Damage is required to update your health value to the correct value. All regeneration heals up to the max_health value.