When using the fertile potion on Nylium it doesn't follow a neat growing pattern, looks like it chooses blocks randomly. Also when you aim the potion to the lateral sides of a block instead of above it doesn't seem to work.
Let me know if this is intended

It basically applies bonemeal to all blocks in 3x3x3 box around the block that's hit - 1 block around it, 2 blocks below and one block up. And as far as I can see it works correctly on Nylium even when it hits a side of a block