When you paste to a combinator that already has values in it, the extra parameters beyond what is currently being pasted are not cleared... this is unexpected behavior, and I believe it should be classified as a bug.
It could be fixed by modifying the loop to set unused parameters to nil. This is slower (by a few ms) than just setting the 4+ entries in the recipe, but since this is an interactive action only initiated by UI interaction (not an automated trigger that could be done thousands of times a second), then the speed is not as important as consistent behavior.
