If you have a origin with a significantly less max health attribute (such as Arachnid, Phantom or Improved Inchling) you lose some portion/percentage of your health every time you rejoin the world.
For example, if your origin lowers your max health to 10 (half of the normal) and if you have 10 HP, you will lose about 5 HP when you rejoin.

Quick reproduction steps:

1. Open a new world.
2. Choose Phantom origin.
3. (Optional) go to a safe spot.
4. Rejoin the same world.
5. You'll see that some part of your HP is just gone.
6. (Optional) go to step 4 for extra frustration.

Minecraft version: 1.21
Fabric Loader version: 0.16.3
Origins version: 1.13.0-alpha.8