I've edited the body of this answer (link to the answer's revision history) twice after ~21 minutes I initially posted it, the second edit ~3 minutes after the first edit, but the 5 minutes grace period didn't apply; in fact the revision history shows a total of three revisions, and two different revisions for the latest two edits, the second revision ~3 minutes after the first revision (revision 3 and revision 2).

What happened to the grace period here?