When I have lit a Nether Portal, (this has happened twice now) the fire was just burning, as I used Flint & Steel, the portal didn't light. The first time it happened, I was in a snow biome, so I thought: 'It must be this snow that makes it not light.' But then it happened on a sunflower plain. Also, I used a full frame. I was on the new update (1.16.1) the second time, but not the first time (I didn't check to see what the first version was; I didn't think it was a bug)