When you grow the pumpkin and melon, it doesn't grow onto the mycelium and moss block. This is a parity issue with Java Edition, where it can grow onto those blocks.
Step to Reproduce
1. Place down a small farm with moss blocks/mycelium surrounding it.
2. Plant some melon/pumpkin seeds.
3. Speed the random tick speed up.
4. See if they grow onto those blocks
---> ❌ It doesn't.
Can confirm in 1.18.2. This is one of those bugs that would be so simple to fix and would provide players with the opportunity to make more lag friendly melon and pumpkin farms using the moss mechanics not sticking to slime and honey, rather than using a million pistons. It may be a parity bug now but Bedrock needs this to be rectified. Plants being able to be places onto mycelium was recently fixed so it would only be natural that this follows.