Unless I have gotten incredibly unlucky, the normal requirements of growing a specific sapling within 2 blocks of a flower on the same y-level do not apply to Mycelium blocks.
MC-298365
Beehives do not spawn on Mycelium
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You’re just unlucky, I got one on my 5th attempt where both the sapling and flower were on mycelium. (Note that the nest is always on the south side of the tree so if you’re facing straight south and then
/filling with air every time you don’t see a nest you’ll be doing that forever.)