Converting between solar-based and lunar-based calendars is fraught with potential for ambiguity. The Buddhist calendar is a solar calendar, while the Hebrew calendar is lunar-based. So converting between dates in the Buddhist calendar and the international-standard (ISO 8601) calendar is typically easy (give or take some subtleties I won't go into for reasons of length). But converting between the Hebrew calendar and the ISO 8601 calendar, or the Buddhist calendar, involves figuring out when the new moon will be — and since the lunar cycle is 29 or 30 days, 12 lunar months add up to 354 days. So the lunar calendars, including the Hebrew calendar, typically add a "leap month" every two or three years in order to track the sidereal year.

All of which means there are many potential ambiguities in converting between calendars, and the combinatorial explosion possible means they probably only want you to convert between non-ISO8601 calendars and ISO8601. It would be too easy to get corner cases wrong otherwise and not notice, I'm sure. So to convert a date from Buddhist calender to Hebrew calender, you'd probably have to do Buddhist -> ISO8601, then ISO8601 -> Hebrew. (I haven't had time to test that for myself yet, I'll post a correction if that turns out to be wrong).

Certainly surprising

One of my favorite interview questions is asking a candidate to, piece meal, build a calendar. They start with Julian, and then write converters to and from other calendars. Any calendar can be converted to any other, by going through Julian

I got the idea from the book "calendrical calculations"

I think this is intentional design. Anyway we can convert `Temporal.PlainDate` to other calendars explicitly (I believe explicitness is good here).

  today.withCalendar('hebrew').toLocaleString("en", { calendar: "hebrew" });
  // "22 Adar 6329"