The command 'mpjunk iVNUM $n' used to junk all the items iVNUM that $n had. It has been changed so that it would only junk ONE item iVNUM.
The only exception is coins: 'mpjunk coins $n' or 'mpjunk platinum $n' will junk the whole pile of coins (or an item with 'platinum' as a keyword if not used carefully).
In order to junk a whole pile, you can use 'mpjunk all.iVNUM $n'.