This is a graphical bug so please read carefully.
This is the bug:
The pipes are shown as if the water is flowing 2 parts to the right pipe path and one to the left.
But in the next tick (ie, when the next water blob comes in from the top), one blob of water magically vanishes in the right path and shows as if the water is split evenly (ie, 1:1 now as opposed to 1:2 splitup shown in above pic)
Attached save file for easier understanding.
Shown in description