JS mw.Title does not normalize input to Unicode NFC form. All MediaWiki titles (and indeed all text) must be in the NFC form. This is enforced by WebRequest (before Title gets the input).
This might be practically impossible to solve. There is String.prototype.normalize(), but isn't not supported widely enough. Implementing Unicode normalization requires 100K+ of data, we probably shouldn't increase the size of mediawiki.Title by that much. (Example JS library that implements this is https://github.com/walling/unorm.)