No chance for mixed rockets?

I know you cannot request mixed rockets from a platform.

So I had an idea: I pre-fill a mixed rocket with the items I want, then I set a request for one of the items. I hoped my prefilled rocket would launch and take all of the items in it into orbit.

The result was not what I expected: The pre-filled rocket launches indeed (instead of bots filling another rocket set to auto), but all the other items remain in the silo - only the requested item is taken into orbit.

So is there any way to automatically launch a mixed rocket?