I just now flagged a particular post as Rude/Abusive. I then had a further detail to report about the post (the username is offensive), but when I went to use the flag dialog again, I found that despite being visibly enabled, I could not actually select the last option for "in need of moderator intervention" to supply a custom message pointing it out:
Here's me trying to at first tap on the selection with my touch screen (some feedback), then clicking it furiously with my mouse:
Usually when Stack Exchange wants to tell me "I can't let you do that, Dave" it'll be pretty explicit about it, and this isn't consistent with that kind of error handling. There were also no console errors.