Can't replicate. Maybe upload a save that showcases the phenomenon?

Something is terribly wrong with that pipe. Pipes are tricky and I never even tried to understand their code so I won't declare that this is a bug nor that it isn't. For the time being all I'm going to say is that this has nothing to do with pressure and it's only the last two or three pipe particles that are bugged. If you remove them and replace them with copies of pipe particles above them with matching colours, they will work fine.

EDIT: Okay it's actually a bug and I know from reliable sources that it'll be fixed in the next version and that it's already fixed in the snapshots. And what I said about it working fine if you remove the last few particles is incorrect; I use the snapshots so that's why that fixed it for me.