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A circuit controlled Splitter set to Set Filter with a static Input Priority (untested with circuit controlled Input Priority), on every belt tier except Basic (15/second), will allow both inputs to flow for the first tick, rather than only the prioritized input, even when provided only 1 tick of control signal. Issue is most noticeable when both inputs are saturated.

Expected behavior under 1 tick conditions: One set of items from the prioritized input

Observed behavior under 1 tick conditions: Two sets of items, one from either inputs

Expected flow behavior while filter is enabled: Prioritized input flows freely

Observed behavior while filter is enabled: Both inputs flow at the same time for the first tick, delaying the prioritized input until the belt ahead has space for another set, then the prioritized input flows freely

Given there is no clear save file provided showing the issue i am going to claim that this is Not a bug due to inherent delay it takes for a circuit network to process signals and take effects.

Isolated the build in editor for logging and save file attachment here

Also a blueprint available here: https://factoriobin.com/post/qpv27i

Anticipated result would be drawing only from the priority belt for all belt tiers with a 1 tick command

Live result in 2.1.8 experimental is currently both belts pulled from at all tiers except yellow

I experimented with this step-by-step quite a bit, and I was not able to find anything "wrong". It looks like the Fast/Express/Turbo splitters internal "buffer positions" are being used to move items directly to the Output lane - they briefly have a content of 22 Items, vs the standing-still 18 Items. This happens during the tick where the Control Behaviour is toggled off by the circuit.

Whereas the yellow Belts/Splitter take an extra Tick to move the items far enough to reach the Internal Buffer; this makes it happen on the next tick where the Control Behaviour has taken effect.