I get that having "competing" inputs (or outpost) that would cause pathing to overlap within the same pipe will result in fluid gridlock. But.... this is just maddening.

Relevant bit is dead center -- I know why the pipe above isn't draining.

https://imgur.com/a/tb8zUBh

Full context: Yeah, I neglected my freshwater reservoir and allowed it to run out while I was tunnel-visioned on something absolutely nowhere near important enough to cause no fewer than 7 cycles of all 7 dupes suddenly forgetting that they'd been housebroken. Note that for whatever reason, the freshwater feed pipe crossed unimpeded right-to-left, with the vertical pipe bridged bottom-to-top. Why it got that spaghetti-fied is a long and boring story. BUT IT WORKED before losing water pressure.

Once I restored the supply, I returned my attention to the [something stupid] I'd originally focused on without verifying that I'd solved the problem. ... yeah ...

When I began troubleshooting, the entire freshwater pipe system was pressurized, including all branches. First, I severed the upstream junctions (seen as gaps near the ladder shaft) and when THAT didn't cure the Ironic Constipation, I deleted the pipe segments AT the input ports of the 3 sinks and 3 shitters. I know that THOSE pipes were pressurized, because I had to mop up the spilled contents. I then rebuilt the pipe segments UNCONNECTED, and then one by one joined the junctions left-to-right (I've procrastinated on installing the Pliers mod...)

So, what the approximate fuck? The toilets drew water, then the sinks drew water, but connecting the sinks (again, left-to-right) created this little siphon-vacuum in the middle. Is this behavior here in my screenshot supposed to be a bug or a feature? Why does a pressurized or partially pressurized pipe, isolated and orphaned from all inputs, still result in a logjam for 6 parallel outputs? What am I missing here?