There are 2 options to workaround this problem at the moment

Grant a Staff Extension

The Staff Extension option will allow you to select an arbitrary student and grant them an extension:

This option currently is not affected by the issue when granting an alternate date that the response to a student request is.

Grant the Proposed Extension Date & then extend

The extension could be granted with the proposed extension date. This will send the student a notification.

However once the extension is granted it can be further modified via the Edit > Grant Extension option in the student's submission:

This can be used to change the extension date further, however no notification will be sent to the student.

If this option is selected it would be appropriate to indicate to the student in the original extension request response that the extension date will change again, and that they can see the date that is in effect via the Assignment page.