I've been playing on a small server on 1.12.2 and today I noticed I was unable to harvest seeds, I downloaded 1.15.2 to test the issue and see if it was only 1.12.2 or if it was multiple versions. Going into a single player world in 1.15.2 I was again, unable to harvest anything from fully grown seeds. This is across all seeds, not just specific ones.

I'm not really sure what you mean by not being able to harvest seeds.