In Safari, using the ethioaa calendar results in "ethiopic-amete-alem" being returned for the resolved calendar rather than "ethioaa".
> new Intl.DateTimeFormat('en-US-u-ca-ethioaa').resolvedOptions().calendar
In Chrome and Firefox, this returns "ethioaa". I believe "ethiopic-amete-alem" is an identifier used by ICU, but the canonical identifier used by CLDR is "ethioaa". Safari seems to be detecting the calendar correctly from the locale, just returning the wrong identifier in resolvedOptions.
This depends on ICU version. And now system ICU version is updated and fixed.