Anonymous user with skip-resolve-name recognized as 'USERNAME' on Windows

MDEV-30487 showed example of missing test case with skip-name-resolve and anonymous user in mtr.
When user connects with anonymous user, connection resolves to USERNAME on Windows only environment, rather to anonymous user.
This situation doesn't occur for non-Windows OS-es.
It may be expected behavior, but I didn't find any reference about in KB.
By testing, this may be related to libmariadb client authentication here where USERNAME is read instead of mysql->user, for which mysql->user[0]='', and I guess because of function read_user_name implementation on Windows, that is using USERNAME.