I'm not sure if this is an intentional feature, but I can't find anything describing something similar online.

I built a portal several blocks lower on the Y axis than an enchantment table (three blocks total between the two; so, from lower Y to upper Y, it went portal, one block of air, one block of polished diorite, then enchantment table) and whenever I traveled through it to the Nether, my character would catch fire.

The enchantment table was on a room/ platform (complete with bookcases surrounding) built above naturally generated dirt ground. Underneath the platform on the ground was the portal, set about two blocks deep into a small pit dug into the dirt

I experienced another common error with this portal, where the portal connects to another existing set in the Nether. It was also catching me fire at this point. I traveled to where the portal should have been coming out in the Nether and built an exit portal there, which connected properly, but I was still catching fire. I tried digging out around the portals (both in the Nether and the Overworld), but couldn't find any lava or netherrack on fire nearby. I tried destroying them both and rebuilding them in the same place, no effect. I moved the one in the Nether a ways away, no effect. When I moved the one in the Overworld away from the enchantment table (probably about 20 blocks or so) it fixed the problem, so the only thing I could think that might be the cause was the enchantment table, but of course that's only a guess.