The GTNH GT5U fork (which more or less copied gt6’s pipe handling wholesale), added the option to force the liquids to go in a specific direction to prevent pipe sloshing. The default behavior is still there, but you can simply wrench them to force them to go only in one direction.
This change has been a thing for more than 5 years, I think.
Since they both share enough of the codebase and how well it works, there’s a good chance it could be implemented in GT6 as well.