I have set up two diff pair physical and spacing constraints and applied them to two different sets of diff pairs. The constraints are numerically the same for the moment. The first set of pairs routed just fine. The second set acts weird. When I click a pin to start the route, it immediately jumps to Neck Mode even though Neck Mode is not selected. Sometimes it jumps to Single Trace Mode when that is not selected. So I route a little ways and neck mode doesn't stop. If I end the net pair by clicking Next, then Done, and then click the slide tool, by just slightly nudging one of the pair's traces, both traces jump to the normal, expected line width. But the starting traces at the pins is all wonky.



Here are a couple of things to check. You can get unexpected results if you leave the diff pair Min Line Spacing set to 0.0



If you have set conflicting Physical and Electrical constraint sets the Electrical constraint set will take precedence. This may also give you unexpected results.

There may be an issue where the constraint set up is not being used as expected. When the software isn't giving the desired results with respect to line width or spacing you should use the Check > Constraints command to query the objects.