steps to reproduce

1 make an old world 

2 load it

3make an obsidian ring 4 blocks wide 5 blocks talll

4 light a fire inside 

expected to happen

you have a magic portal to the nether

what happened  a fire boring

it did not light 

i did a command /setblock ~ ~ ~ portal and went to the nether(it was infinite not a big surprise though, the flat world does not have a flat nether) i went back to see if it lit my other portal and i was stuck in nothing so my brother joined and i teleported to him  and left made it infinite now fine