Reducing the player's base max health using /attribute does not always reduce their current health

The bug

When the player's base max health is reduced using /attribute, their current health (stored on the server) is not always reduced accordingly. (There have been reports in the past that this bug occurs whenever the player's total max health is reduced, including using attribute modifiers, but as of the 1.20.2 snapshots I can only reproduce this by modifying the base value using /attribute.)

Reproduction

Setup

This bug occurs inconsistently and can be hard to detect when it occurs unless you're paying close attention, so it's preferable to have a dedicated testing setup. Let's start with a few of commands:

/gamerule doImmediateRespawn true
/gamerule keepInventory true
/gamerule naturalRegeneration false
/spawnpoint @s ~ ~ ~

Now let's make our current health value visible in the sidebar:

/scoreboard objectives add health health
/scoreboard objectives setdisplay sidebar health
/kill @s

And, for extra visibility, let's make particles emanate from ourselves when our current health exceeds our max health. First we run the following:

/scoreboard objectives add temp dummy

Then we put the following in a powered repeating command block:

execute as @p store result score @s temp run attribute @s minecraft:generic.max_health get 1.0

And the following in an unconditional, always-active chain command block pointing away from the repeating one:

execute as @p at @s if score @s health > @s temp run particle minecraft:glow ~ ~ ~ 0.2 0 0.2 0.2 10

Now let's set up some impulse command blocks which can cause the bug to occur. Place a button on each one, and make sure they aren't adjacent to each other.

attribute @p minecraft:generic.max_health base set 1
attribute @p minecraft:generic.max_health base set 20
attribute @p minecraft:generic.max_health base set 30

Finally, we need a command block to heal ourselves to full in case we've failed to repro and want to try again. Place a button on this one too.

effect give @p minecraft:instant_health 1 20 true

Reproducing the bug

Go into survival mode. Using the command blocks you just set up, set your base max health to 30 and heal yourself to full. Try reducing your max health, first to 20, then to 1. If you get particles and notice that the health value in the sidebar exceeds the hearts on your screen, the bug has occurred! If not, start over from the beginning of this paragraph.

Once you've repro'd successfully, set your max health to 30 again. As the particles cease, note how while you do regain some black hearts, the amount of red hearts remains unchanged. I assume this is because the client is sent an "update max health" packet but the server sees no need to send an "update current health" packet since it did not know the client was out of sync.

Being hurt or healed while the bug is occurring will fix it.

Remarks

NBT state has no effect

Due to MC-197818, the player's NBT does not accurately reflect the state of their attributes. This does not seem to affect whether or not the bug occurs at a particular point in time, but if desired, you can run /kill @s to trigger that issue and MC-179940, removing the max health attribute from your NBT and starting over again with 20 health.

Attribute modifiers have no effect (anymore)

Some past reports indicate that it was possible to get the player's current health to be higher than their max health using attribute modifiers. I tried to reproduce this by playing around with the following items during the tests mentioned above, but ultimately I did not encounter any inexplicable behavior.

/give @s stick{AttributeModifiers:[{AttributeName:"minecraft:generic.max_health",Name:"health-",Slot:"mainhand",Amount:-10,Operation:0,UUID:[I;0,1111,0,111111]}],display:{Name:'"Health Reduction"'}}
/give @s blaze_rod{AttributeModifiers:[{AttributeName:"minecraft:generic.max_health",Name:"health+",Slot:"mainhand",Amount:10,Operation:0,UUID:[I;0,1111,0,111112]}],display:{Name:'"Health Boost"'}}
/give @s golden_helmet{AttributeModifiers:[{AttributeName:"minecraft:generic.max_health",Name:"health++",Slot:"head",Amount:20,Operation:0,UUID:[I;0,1111,0,111113]}],display:{Name:'"Mega Health Boost"'}}