What should I do if my re-entry permit is valid until an earlier date than my extension approval?

Story not really clear to me, as I can see two different ways in what happened.

Case 1, You have an Extension of Stay based on being married and permitted up to 20 Feb 2020, You also have an Re-Entry permit valid until 20 Feb 2020. You went on 20 Jan to the Immigration Office and applied for a 1 year Extension of Stay based on marriage and you got a new stamp 30 days under consideration with report date 20 March 2020. In this case all correct, your Re-Entry Permit keeps the same valid until date and can not be changed when you apply for new Extension of Stay.

Case 2, You visited on 20 Jan 2020 the Immigration Office, applied for the 1 year Extension of Stay, got the 30 day under consideration stamp with report date 20 March 2020. After this is stamped in your passport you applied for a Re-Entry stamp and the officers stamped valid until 20 Feb 2020, which in this case should be 20 March 2020. You have to go back and get this corrected.

Go back to immigration to have it fixed. Advise is always to check your passport when you receive it back and ask questions immediately if you are not sure.