So, judging by the code for ICrop#getSecondaryChange() I suspect that the secondary chance for dropping another seed and item when harvesting a crop should increase to 20% when it's planted on farmland of the same tier. However, this does not happen (at least in version 1.15.2-3.0.1).

After doing some digging I suspect that this is due to how CropTier#isEffectiveFarmland() checks whether the crop and the farmland are of the same tier. In particular, this.farmland is of type Supplier<? extends FarmBlock>, whereas block is of type Block. Please correct me if I'm wrong, but this should never type check, right?