Observing this setup tick-by-tick with F4 option show-transport-lines, I cannot see anything wrong:
Stack Inserters each pick-up 16 Items from the Infinity Chest
07-14-2026, 08-06-20.png (156.17 KiB) Viewed 184 times
11 Ticks waiting for the Arms to swing
Upon reaching the drop position they immediately drop 4 items onto the Input side of the Splitter
07-14-2026, 08-13-45.png (145.4 KiB) Viewed 184 times
Splitter moves forward, items begin to peek out of the Output side. 4 more items are dropped onto the Input side.
07-14-2026, 08-15-25.png (146.35 KiB) Viewed 184 times
Splitter moves forward again, Items are visibile but still "in" the Splitter. At this point, the Output Priority side of the Splitter is now "full", waiting for the Transport Line to move items onto the Belt. 4 more items are dropped onto the Input side.
07-14-2026, 08-16-01.png (143.01 KiB) Viewed 184 times
Splitter moves forward, with the Items from the previous step going to the non-blocked Output side. Last 4 items are dropped to the Input.
07-14-2026, 08-20-37.png (140.34 KiB) Viewed 184 times
Inserters swing back towards the Pickup point, Transport lines advance, moving the first 4 items onto the Belt. There is enough room for the last 4 items to go to the Priority Output again
07-14-2026, 08-21-26.png (138.21 KiB) Viewed 184 times
Splitter Output Priority is just that: a Priority. Oversaturating the Inputs will cause it to divert to the non-Priority Side.
For comparison, the Inserter-to-Belt case takes 1 extra Tick to unload while it waits for the Transport Lines to advance, creating enough room for the 3rd/4th set of 4 Items to be dropped.
Good Luck!
Re: [2.1.10] Output priority splitter not working when inserting directly