a) Your method of giving grass drops appears to rely on a player-driven even rather than the harvestdrops event -- this means that using a bucket of water on grass, for example, results in the grass being destroyed and nothing dropping from anything. (Whereas using a bucket of water in a vanilla environment results in seeds being dropped.)

b) Your method of giving grass drops appears to conflict with other mods that don't also use the same method to drop seeds. Specifically, Mystical World uses LootTable injection into the block drops for the Grass in order to inject its aubergine: I spent around 5-10 minutes attempting to find aubergine seeds by destroying grass in a modpack with Simple Farming & Mystical World, but they didn't seem to drop.

Whereas, seeds from TofuCraft Reloaded and Druidcraft, which I know both use the same block-break method, did drop.

I suspect that your block-break event is prevent the normal loot table event from being fired.

Obviously, there are downsides to the LootTable injection: it should be possible to fine-tune it more so that you don't get it doing draws from every pool injected -- it would possibly be better to manually inject the items into the main pool so that it becomes a chance to pick an item from the list of seeds.

Obviously the best solution would be for Forge to provide an adequate implementation.