liquid flow stops in the middle of non-branching pipe

I have an array of pipes flowing from valves set to 400g/s to outlets. The pipes do not branch in between. The pipes carry crude oil. At apparantly random flow stops in some of the pipes and the crude oil starts to accumulate. Eventually flow stops completely. See pictures for illustration. The pictures are the same segment with a few cycles passed between them. Note how oil accumulates in the middle of the fifth and tenth pipe.

This happens in this build regularly. It is not cleared by reloading the save/restarting the game. It can be cleared by removing and rebuilding the pipe but it reoccurs within a few minutes.

swanky pigpen bug report.sav