Hemp can grow into any block directly above it, since there is no isAir (or at least isReplaceable) check in the canGrowTop function - https://github.com/BluSunrize/ImmersiveEngineering/blob/1.20.4/src/main/java/blusunrize/immersiveengineering/common/blocks/plant/HempBlock.java#L149

This is also the case in 1.20.1, based on the code, but is not the case in 1.19.2 and earlier.

I may have used the suggestion form out of laziness™ (but also because the bug template doesn't even let me select 1.20.4)