The bug

Not a duplicate of MC-170828, or even MC-177870.

After standing on soul fire for 3 seconds or less, you are still on fire, but the damage is not doubled.

To reproduce

1. Use a flint and steel on soul sand/soil to create soul fire
2. Stand on soul fire for 3 seconds
3. Walk away from the soul fire
   → You'll continue burning, but the damage isn't doubled as you would expected. (Half a heart damage)