When merging stacks it merges all of a block material, no matter the shape! (advise NOT to use the merge tool in the inventory)
A similar issue when crafting material blocks is that if you have one in the toolbar, no matter the shape you chose the number you craft will be added to the number in the toolbar (current fix.. move blocks out of toolbar to stop the crafting auto merge)
If block shape could be determined after crafting then this would also sort the problem (now I have 270+ inverted arch I dont need, instead of a mix of cubes and slopes too)