I use https://www.yellowbridge.com/chinese/dictionary.php as my primary dictionary. For 用, and any character whose 拼音 would be "yong," the given 注音 on this site is always ㄧㄨㄥ. However, when inputting characters on a 注音 keyboard, it does not accept ㄧㄨㄥ, but rather ㄩㄥ. Additionally, a Taiwanese friend has told me that there is no such thing as ㄧㄨㄥ.
So is ㄧㄨㄥ just a weird alternate (but still correct) rendering of the sound in 注音? Or is this dictionary site in error, and it actually should always be ㄩㄥ?