The bug

When max health is reduced (e.g. when you wear or hold an item which increases your maximum health with the generic.max_health attribute modifier, and then stop wearing or holding that item), your maximum health value is adjusted, but your current health value is not.

It can therefore happen that health > max health.

Reproduction steps

1. Create a health scoreboard

   /scoreboard objectives add Health health
   

2. Set the scoreboard to display on the sidebar

   /scoreboard objectives setdisplay sidebar Health
   

3. Switch to Survival mode

   /gamemode survival
   

4. Give yourself an item with the generic.max_health attribute and select it

   /give @s dirt{AttributeModifiers:[{AttributeName:"minecraft:generic.max_health",Name:"Health boost",Amount:40,Operation:0,UUID:[I;0,1111,0,111112]}],display:{Name:'"Health boost"'}}
   

5. Give yourself regeneration so that the scoreboard updates and your Health is set to 60

   /effect give @s minecraft:regeneration 4 255
   

6. Your Health score should now be 60
7. Now deselect the item
   → Notice how your Health score is still 60.

The game will change your health back to 20 as soon as you get damaged, but does NOT reduce your health lower than 20 because of the damage.
If the difference between max health and health is large (e.g. above 20 in this case), you die instantly ("You Died!" screen appears) but are not actually dead yet, see also MC-19690.