Mono uses embedded culture data in form of culture-info-tables.h. The code that populates the managed data from it gets two things wrong:

• It ignores the calendar id and only considers the locale. So, for Hebrew locale and Hebrew calendar you'd still get resources for the Hebrew locale and Gregorian calendar. Naturally this results in incorrect month names and other inaccuracies.
• Leap month names are never filled in. This causes the managed code to assume that the leap month names are identical to the regular ones. Consequence is that 13th month in a leap year has no name and round-tripping of DateTimes fails.

The code around ves_icall_System_Globalization_CalendarData_fill_calendar_data will likely need to be refactored to fix the problem.

On September 1, 2019 this broke the CI on xunit tests that exercise the round-tripping of current date in the Hebrew calendar.