I have the same problem in version 6.58.
I set up a system with 760 tables, 2 Screens. On several tables, the Backglass is covered by a black screen and only the DMD appears. If I check the option "Form to Top" on the backglass, the Backglass appears, but it goes in front of the DMD. I noticed that the Playfields and Backglass with Videos are not displayed. On another computer I use PinballX 6.10 and I don't have these problems. But unfortunately I can't find the installer to download.
Is there a repository with older installers?