Screenshot
The problem is that it breaks if the other half of the underground belt is saturated while it is working. You can see it in this clip:
webm
When the design works properly, the top and bottom stack inserters are always activated in sync, activating the same number of times. This is in part because the top inserter lets items go through slowly while it's inserting, leaving space for the bottom one to insert its items right after.You can see that when I saturate the other side of the belt, the top inserter inserts instantly instead, completely stopping the flow of items from behind and preventing the bottom inserter from going as frequently as it should. This results in the top inserter activating almost twice as often as the bottom one, and the design not working properly anymore.
I'm not sure which scenario is the intended behavior, but either way it's currently inconsistent and unreliable.
Blueprint String
Edit: I've also done the test without any mods just in case, the observed behavior was exactly the same.
