Before Reading this, please note that the version I am using is Optifine 1.16.4, so this may be patched in later versions

If the Ender Dragon is at any level of health, leaving and rejoining the game resets the health of the dragon. The Images below show before and after leaving the game (Bossbar and Bossbar 2 respectively)

You can recreate this by going to the end, damaging the Ender Dragon, leaving the game, then Rejoining the game. Notice how the health bar is now back to full health