You have my sympathy. I went through 'trial by fire' with the display manager/ object display/ style display.
Interesting that it's both valves and pipe that are affected.
My first thought was that the pipe System is defined as single-line, so the valves would be single-line, too.
It could also be display related. When you cut a 2D section, you have a choice of display representation. (eg "MEP-Section and Elevation"). I presume it would be the same case with 3d sections. That's the Display Set you need to identify & locate in Display Manager, and add a 'two-line' tick against Mvparts. You may need to deselect whatever is currently enabled, BUT tread carefully! I've had a situation where I had the schematic and the model view of the valves displaying at the same time. I've also made all the valves disappear.
This will work unless the Style or the specific Objects have display overrides. I think.
Possibly, your pipework is trying to connect to a 'plumbing' connection and transforming itself into crappy 2d Plumbing line. Also, possibly, your affected pipe uses a different System Definition and goes single-line below a certain size.
Finally, you could save a lot of ink by making your flanges, levers, etc. plot in a thin line! Good excuse to decrypt the Display manager.
But you do have the coolest North arrow. I suppose being the North Pole and all...
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