Ok, so I was playing in my Survival World, and I had this issue when my Nether Portal burns me, when the nearest fire is 1 block lower than the portal itself. My portal is a 4x4 on both sides.

Steps to Reproduce (keep in mind, this doesn't work all the time, it only happens randomly)
1. Put a Nether Portal in your desired location.
2. Put a piece of fire next to the portal (MUST BE 1 BLOCK LOWER THAN THE PORTAL)
3. Enter the Portal
4. You should burn randomly, despite the fire you placed 1 block lower than the portal itself.