Are you using same fuel pumps? Because I have experienced both behaviours on both cards. I presume it’s just the way transactions are presented to monzo.
If the pump actually charges you and then refunds you, you will see this in app automatically. If the pump authorises the high amount but then just presents the actual, lower cost. How can monzo know until they receive the presentment? In that case, I’m not sure they can do much. Would still be great to get a notification when transaction settles for a different amount.