Adding events not working when Calendar converted from java.time

[MrStahlfelge]

Event added in the following way is not showing:

val localDate = LocalDate.now() // java.time
val calendar = GregorianCalendar.from(atStartOfDay(ZoneId.systemDefault()))
val calendarDay = CalendarDay(calendar)
calendarDays.add(event)

It is showing if it is added the following way:

val localDate = LocalDate.now() // java.time
val calendar = Calendar.getInstance().apply { time = Date.from(atStartOfDay().toInstant(ZoneOffset.UTC)) }
val calendarDay = CalendarDay(calendar)
calendarDays.add(event)