The wiki for minecraft does not have a height requirement but when I try to spawn blazes at Y=26 then nothing happens but when I do exactly the same at Y=65 it works fine.
I made my two tests in a similar way. Just a simple platform almost two high (trapdoor on top to prevent pigmen). Then I walk away 24 blocks and look at the platform.
The only major difference is that the one at y=26 cuts through a nether fortress pillar, and the one at Y=65 is on top of the fortress.
The map I use is skyblock 4, at position X=-80 Z=80 chunk. I don't know if that matters.
Best Answer
I figured it out, the fortress bounding box just didn't go down below 46.
I downloaded a bounding box mod and could see the spawn areas.