Package edit UI says "slug is valid" but then fails validation on submit

If I save a public package but give it a slug name like test-ext@bdw429s the validation on the UI says "slug is valid" but when I submit the form, it errors with the message:

Slug must contain only numbers, letters, periods, underscores, or hyphens and must start with a letter.

I found this when editing a previously private slug back to being public. While I was ultimately prevented from inputting incorrect data, we probably shouldn't give the message "slug is valid" if it isn't.