As per some recent comments on this thread, customers have confirmed that the issue is fixed.
We are closing the thread. In case issue is encountered again, please log a new thread.
-InDesign TeamTim commented
Same issue for me.
When I quit and restart InDesign multiple times it occasionally starts with the panels visible but then the master pages and document pages are hidden under the frame and are not accessible. VERY frustrating.
macOS High Sierra 10.13.4 / 13.1