Thank you for the explanation. I will try to find a work around for my situation.

How is this not a bug if the 'Select Input' does update the signal count? What makes this different?

Selecter combinators 'Random Input' doesn't update signal count until it selects a new input. This causes my inserter to not have the actual stack size derived from the signal count and overfill the train wagon. I expected that it only selects a single signal to send through instead of creating a ...

I have also tried for this achievement where I placed a whole stack in the rocken and delivered cargo to Orbit but did not get the achievement, neither on steam nor in game.