The bug
When a player's health goes over 42HP (21 hearts), the tab list displays the player's health in fractional hearts but says it is "hp"; e.g., 22 hearts of health (44HP) is displayed in the tab list as "22hp". This is confusing as "HP" refers to half-hearts of damage/health, not hearts.
How to reproduce
Open to LAN
Type
/scoreboard objectives add health healthType
/scoreboard objectives setdisplay list healthGo into Survival
Type
/effect give @s minecraft:absorption 30 5Press tab
Type
/effect give @s poisonHold Tab and watch as your health goes down.
Can confirm in 20w51a.