what I am suspecting, Asus must have taken down the file from its server due to known bugs they discovered just after pushing the version over the Air. Thats why its getting detected in the system update but not getting downloaded since the file has been removed from the server. but still, Asus must come up with an official statement regarding the same.
since there is a problem with the update, we should wait for a fix from Asus and go for only FOTA update instead of manually updating to the .27 version. They have taken down the FOTA file because there's must be some issues which they detected, thats why they took it down.