Huh?
What are you looking at? The only year Frazier's defense wasn't top 5 with Allen was in 2020 when it was ranked 14th.
In the other years it was ranked 2nd, 3rd, 1st, and 4th.
In PPG starting in 2018 it was ranked 18th, 2nd, 16th, 1st and 2nd.
Bills were literally a "no-fly" zone under Frazier...over a 4 year period the Bills had more games they allowed under 200 yards passing than they allowed over 200 yards. In an era where 300 and even 400 yard games are common, that's ridiculous.
The problem was mostly in the playoffs.