Copy type A block to positive pattern, place bits of type B block, apply pattern to type B bits with one stack of type A bits in inventory while holding a bit bag. Application of pattern does not remove type A bits from inventory, and you can continue applying the pattern in this way to generate type A blocks with no cost.

Does not seem to depend on block type, as long as you have a single stack of the bits in your inventory and are using a different block's bits as your target for the pattern.

Bug occurred in chisels and bits version 8.4 while running the following mods with forge 11.15.1.1763
JEI, StorageDrawers, Thaumcraft, Mapwriter, Chameleon

Fixed, a new build will be out shortly.