Masking regions that are part of a nested family (such as a valve symbol) appear correctly on the screen, however do not mask when printed.
The Autodesk solution was to print everything as a raster image instead of vector, however print quality, especially PDF quality is extremely reduced and this is not an acceptable in a professional environment.
This issue has been going on for several years and documented may times on this forum.
Please resolve this issue and provide a fix for the past several Revit releases.