Strange splitter behaviour with invalid input

[Nerby]

I have just discovered a compatibility problem between Pyanodon's Wood Processing Unit MK1 and Loaders Modernized unloader (which I already reported to both).

The bug is that if I put an unloader at the direct output of the wood processing unit, I end up having 12 items in a single belt section instead of 8, as it can be seen here:

Loads on the two belts after the splitter

Screenshot 2026-10-05 132652.png (2.63 MiB) Viewed 170 times

The funny thing about the splitter (I cannot classify it as a bug, because the one of the splitter inputs is invalid) is that it does not split the inputs on the output lanes: the overloaded belt remains on the same side, as shown here:

Loads on the two belts before the splitter

Screenshot 2026-10-05 132846.png (2.42 MiB) Viewed 170 times

[Nerby]

There is a possible explanation by kryojenik (Loaders Modernized author) at https://mods.factorio.com/mod/loaders-m ... f0263fc510.
However, that doesn't explain the strange behavior that I show here.

[kryojenik]

The splitters input is not invalid. The right lane of the left belt is a stack of 2 before your splitter (zoom in close). This is the stacked output from the Wood processing unit. The left lane of that belt is a single stack from the non-stacking loader output.

How splitters handle full belt lanes is discussed elsewhere (viewtopic.php?p=585390&hilit=splitter+m ... es#p585390). So, the output from the splitters is also behaving as expected.