Having a high pass completion isn't the same as playing well.
AWB has pretty consistently had a good pass completion rate. 84.2% this season and 86% and 81.2% on the previous seasons. Or ranging between 79th and 96th percentile for fullbacks, if you prefer.
The problem is that he gets that high passing rate even with his technical limitations by being very conservative on the ball. Over those same three seasons he was 21st-66th percentile for progressive passes, 34th-63rd for progressive passing distance, 48th-58th for passes under pressure, 34th-64th for passes into the final third, etc.
Whereas someone like Dalot (who really isn't all that good a fullback himself) has been 91st percentile for progressive passes, 87th for progressive passing distance, 85th for passes under pressure, 80th for passes into the final third, etc. He has also played more through-balls this season than AWB managed in the last three combined.
In other words rather than giving the ball away constantly, the problem with AWB is that he's a cul-de-sac in posession. When the ball gets to him he struggles to actually move it further up the pitch or handle being put under pressure, so the ball inevitably tends to go sideways or backwards as that's how he can avoid surrendering possesion. Which helps cripple our right flank in possession, because we struggle to actually progress up the pitch down that side.