Menu Maintenance is broken for several DLLs where you cannot apply customization - versions 200.8 - 200.11. The issue has been fixed in 200.12 under ERPS-95829 and there is a HotFix available from support.
Some of the Epicor UI DLLs contain references for multiple different Forms. Some of those Forms are internally addressed (not available for adding to a menu) and some can be externally addressed as the Form to load when a menu option is selected. Since the Customizations are keyed by Form name and not DLL reference, Menu Maintenance needs to know which Forms are “Externally” available in order to show the Customizations in the dropdown Combo. A change made in 200.8 was designed to address a problem with displaying the appropriate customizations and while that update corrected the initial reported problem, it caused a new problem for a handful of other DLLs / Forms.