I repaired and activated a ruined portal near my base(Survival mode, normal difficulty, world created on 1.17.20.23 beta) . 

When I passed through, I was not taken to a portal within a nether fortress, but rather a generated portal within a lava flow.

This led to two problems, the trivial one was being  stuck(solved by taking stone through and bridging over the lava), and the main one being that whenever I enter the Nether, I take damage from being lit on fire.  This can't be mitigated, because it happens while the nether loads (you can hear your character taking damage even on the loading screen). 

I don't know how one can reproduce this consistently - I'll get the seed and post it, along with coordinates of the nether portals.

The lava flow is very shallow, so the placement might just be phenomenally bad luck(the portal being generated on solid ground when the nether was initially generated, with the lava flowing around it a few ticks later).

Taking hits while loading is definitely a bug though.