I tried to recreate this issue.
In your families, find your Connection family that you use for your moment frame designations. Right-click and edit the family. Make sure that the fill pattern for the filled region is Solid Black. Also make sure that the actual selected color for the filled region actually is black (and not the pale gray.) Load back into project and see if it's resolved.

When trying to recreate the issue, I changed the filled region Solid Black to grey in my moment connection family. On plan, it immediately showed as grey. But when I panned around, my moment connection symbols turned black. Panning or zooming, they would temporarily show as grey, then again turn to black. But a print preview showed them as a screened grey. This is why I'm suggesting to make absolutely sure that the filled region in your moment connection family is set to black and is actually using a black color.

It works, now the drawings looks nice! Thank you!