Stoglim
Comments
Unsure if this is related, but I experienced similar:
While mapping a Nether Portal to an overworld portal, I created a portal (in the Nether) over a lava ocean. I started by creating a platform (replacing lava with Nether Brick Blocks) and put the base of the Nether portal in that base level. Every time I entered the portal, I took fire damage. I corrected for by raising the portal one block.
I am curious if your lava-flow is a red-herring and there may be lava directly below the obsidian in your Nether portal. (which is probably still a bug to take damage from it).
It appears these portals are all blocked to some degree on one side. In order to exit properly, the portals need air around the portal surface.
Experienced this in 1.17.11.01 Bedrock Dedicated Server.