Usually Charlie Austin is talking out of his ass, but our right back position is definitely a weak spot. Just the fact he's saying it after playing him last week shows opposition players know that and are targeting him. Wan-Bissaka has not been as good defensively as he was when he first came and you can always rely on him to lose his man at the back post and his attacking play, while better, is still most of the time non-existent.
I think you're speaking a lot of sense. However, where I disagree with you is on your statement Wan-Bissaka has gone backwards defensively.
Wan-Bissaka has not gone backwards defensively, he just hasn't gone forward defensively as much as we would like.
From day one Wan-Bissaka has been brilliant in 1v1 duels on the ground. Wan-Bissaka remains brilliant against a player trying to stand him up and go past him. Where he lets himself down is in his general defensive positioning, especially when the ball is on the opposite flank and/or when the ball is in the air. He routinely gets caught out by the flight of ball into the box and, as you say, this is something teams look to exploit. People are aware that if you keep slinging the ball into our right hand channel, eventually, Wan-Bissaka will give up a chance. Probably several.
This is not a change in his game. He's been this way since we signed him. However, I am sure the coaches would have expected him to grow in these areas over the years he's been a United player.
Wan-Bissaka definitely can defend but there are holes in his game.