Force to UTF-8 option problem

I use Ultimate Member most recent alone (without any add-ons)
I encountered the following problem:

if user fills the Registration form in UTF-8 with non-English characters (e.g. Cyrillic) then the user’s display name (in profile) is garbage (????? ?????). This is because user_meta[‘first_name’\ and user_meta[‘last_name’} are garbage, and user meta is garbage because:

function um_force_utf8_string( $value ) invokes $utf8_decoded_value = utf8_decode( $value ); which does not (see PHP documentation) convert any UTF-8 character properly. Thus, if “Force Strings to UTF-8 Encoding” option is “On”, and the $value is in UTF-8 already (e.g. it contains value of the ‘first_name’ field of the registration form) then utf8_decode may return garbage, and um_force_utf8_string will return the garbage too.

Regards,
Michael.