Because of a quest system i played around with the different oreChunk settings.
With useOreChunks = false punching dirt dropped two dirt. Using a Silktouch Pickaxe on Stone dropped one stone and one cobblestone.
Found it in your onHarvest Method, If !useOreChunks:

1. there is no skill test for found ores
2. if its not an ore, i.e. !foundBlock, you add a damaged stack on top (which explains the silk touch behavior)