I did a couple of runs using the default numbers, delaying the retirement date by one year, and my success rate increased as expected.
Maybe what you are seeing - which isn't what you would expect - has something to do with your specific set of inputs. Have you tried playing around with your inputs (SS, AA, etc.) one at a time and testing 2020 and 2021 success rates to see if that will identify the source of the decline?
Numbers is hard.
Retired in 2005 at age 58, no pension