Was messing about with a datapack making blazes set fire to things they walk over when I noticed that they didn't do it in certain areas of the nether and after a bit of investigating I found that some caves in the nether don't have air.

I used the command /fill to fill in a 20x20 with glass replacing only air and when doing so in some caves if wouldn't fill in the blocks.
after I placed a few blocks and broke them again, then ran the command, the blocks in the positions I placed things were then filled with glass

Also I stood in the places effected and ran an execute if block ~ ~ ~ air and got a "test failed" in response, whereas if I did it elsewhere it confirmed that the block was air