When bone-mealing fungi in order to create huge fungi, the fungi itself generate on the lowest Nylium block (Warped or Crimson). If there is no Nylium block under the fungi, then it will never grow (which could be intentional design). The Fungi itself can actually stay in its place if it is not deleted by one of the Huge Fungi stems.

How to Replicate

Place a Nylium block and a couple of block above it place any type of dirt block. On the higher block plant a fungi and try bone-mealing it. The Huge Fungi will generate above the Nylium block instead of above the fungi.