The small pieces of ores are cheaper than the ore itself, then I can exchange the ore in the table and then take out more pieces......... then I'll hv infinity EMC, is this a bug or a feature?

Depending on what type of mods along side it, the devs physically cant fix it because you can dupe emc by just going to a village and getting a leatherworker. then trading leather to it for free profit.

Depends on the Modpack and has nothing to do with this Mod
Its a Modpack issue