I think that has to do with your source point. Source should be the lowpoint of the sanitary system. On vent I make it the high point.

Where is your source point on the affected p-traps?

It not only has to do with the Source Point but the direction of takeoff for the P-Trap. For Desig Line you want the direction in to be from the top of the P-Trap and not the leg. With the attacher arrow it would be reversed with the direction in from the leg. We use the P-Trap with Design Line and the attacher arrow so I have a button script set to "takeoff by other end" set as the default.

It depends on the design line source along with flow direction.

Following two items will impact the direction of the P-Trap.

• The location of inlet & outlet of the P-Trap. the settings should make the connection indicators the same with 2nd image.

• The setting of the service. It must be set as "Extract" for the drainage service.

When both above settings are correct, the behavior of the P-Trap will be correct. You could check if the P-Trap with incorrect behavior has the incorrect settings.

Hope my reply will be helpful.