This logic is inconsistent with their Overworld counterparts. In this image, the block directly below should probably be normal netherrack.
Affects both grown funghi and naturally generated fungi.
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Relates to MC-171517

Can still reproduce in 20w09a with growing fungi, please reopen
EDIT: naturally generated aren't fixed either.

I disagree with the reopened of this issue I have waiting a long time and the nylium have been converted into netherrack when a large fungi is planted on it.

Then why in my testing do freshly generated fungi still have nylium beneath and freshly bonemealed fungi the same?

Naturally generated nylium is still not fixed I agree with that but when bonemealed fungi I have waiting a very long time and it that worked.

Resolved the report again, nylium converts to netherrack, however not immediately which is now covered by MC-174164.