When the user deletes text inside of a marker, WebKit shrinks the marker, but when the user inserts text inside markers, they do not grow. <rdar://problem/9594282>
Hi Justin, Could you give more detail for reproduction? Usually, the spelling/grammar markers are removed when the caret is on the marked range. So user can't remove the marker because there is no marker under the caret. I guess your are talking about different type of marker.
There may be some real issue here, but we aren’t going to keep this bug open.