A better example would be this, because your example script doesn't use any inputs.
I'm so confused by that example. There shouldn't be any implicit substitution into the inner ring because it has an explicit formal parameter. So the only substitution in that whole expression should be the sprites into the first input to ASK. And I have no idea where that error message came from.
I was confirming that blocks that have with inputs variadic input, does not get filled. A much better way to show this would probably be
Well, as I said, I think that the specific behavior you want to call "variadic," which happens when a variadic input has no visible slots, should happen only when the entire expression inside the ring has no visible slots, and (it just now occurs to me) only when there is just one such zero-slot variadic input in the expression.
know*
(sorry, this misspelling triggers me A LOT when i see it)
I noticed a commit in dev which changes the broadcast behaviour and using ego-lay_atman-bay original project now gives these results
Simple calling broadcast with one input works fine - "test" is only inserted into the one visible empty slot
These ones silently fail - I think they should maybe error saying x input(s) required x + 1 given, but a silent fail is better than doing something unexpected
[edit 3 hours later - Jens has completely removed giving out error messages for mismatched argument numbers so silent failing is now the way things are 
]
I even tried this and it did exactly as I'd expect and want
