If you have an under consideration stamp, it means it is in "process" and you have to return to complete the second stamp on or after that date.
If it is still in process and the date that is stamped under consideration passes, You are not on overstay.
It remains in process, in limbo so to speak.
You do need to get the second stamp completed sooner than later of course.
As you have been granted time on the extension already waiting for it to process, there is extremely rarely any reason for the second stamp to be denied ever.