You’re really misunderstanding things here. What you are doing is not adding an “audio cross fade” but a video Cross Fade video transition, which does an automatic crossfade on the audio, also, which can not be manipulated independently of the video except in the Inspector. Thus, there’s no fade handles the command you are accessing via the menu can affect, as stated, you’re applying the default VIDEO Cross Fade, which fades audio automatically, it does not use fade handles. The menu option you’re trying to access via the menu only affects fade handles. You have to manipulate the fade handles first, to use that option.
If you’re trying to do an audio only transition, you’re going about it totally wrong. There is no bug here, you’re simply using the wrong tools.
1. Double click the audio portion of a clip to expand the video and audio of that clip so you can trim the video and audio independently of each other.
2. Extend the audio portion if desired, adjust fade handle as desired.
And yes, this has been a huge weakness in FCPX for 12 years now that Apple thinks is unimportant.