Fixed in the next release. This issue was a divergence in behavior between vBulletin and XenForo.
In vBulletin, the expected behavior (as seen with regular posts) is to show a preview / errors when going from quick reply to the full editor.
However in XenForo, the expected behavior is simply to open the full editor without showing preview / errors.
Additionally, XenForo has another different behavior from vBulletin: In vBulletin, 'required field' errors only occur if the $dm->set call was never made for the field. However, in XenForo, a 'required field' error occurs when a $dm->set call is never made, or if the call sets an empty value, such as 0, NULL, or "" (there are cases where these values are valid), or if the call results in an error. In the final case, the error becomes redundant with the 'required field' error -- of course the required field is missing, because the value did not pass validation.
This second issue is fixed in the next release by making some modifications to the base XenForo DM to prevent 'required field' errors if $dm->set was already called for that field.