I think the best way to describe the bug is by showing it...

Not big issue, just a bit surprised that this is possible.

I know, this is a vanilla bug when you put a non-full block in soul fire tag (that's ridiculous but I can't do anything)
https://bugs.mojang.com/browse/MC-214503

Got an idea how maybe it could be fixed, but I don't know how to write such a fix as I don't know how to code:

Maybe you could exclude non-full blocks out of the tag... and then if a game would try to set this block on fire and it's those slabs or stairs in specific orientation (slabs being on top-half of block, stairs being basically upside-down), then mod would attempt to replace the normal fire with soulfire one.

You could add a new tag which would be specifically for soul variants of slabs and stairs, so the mod would check for blocks in that tag.

Of course we hope that Mojang will fix this vanilla bug some day, but in the meanwhile that's the best I can think of...

Yes isn't this basically VazkiiMods/Quark#2711?