2.1.42 [ECMA-262/9:2018] Section 220.127.116.11 String.prototype.toUpperCase ( )
V0185: Only characters in the Basic Multilingual Plane are converted to uppercase
The specification states:
... String.prototype.toUpperCase ( ) This function interprets a String value as a sequence of UTF-16 encoded code points, as described in 6.1.4. This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Only those characters in the Basic Multilingual Plane (values no greater than 0xFFFF) are converted to uppercase. Others are left unchanged.