Verified with Chrome and Safari.
If you type wikitext ( I tried with * ) and then delete it, the wikitext warning won't go away automatically.
If you click on it to make it disappear, proceed to add more wikitext and delete this again, then the box will auto-hide correctly.

(Two square parentheses don't seem to trigger the warning at all?)

Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=51701