The goal is straightforward to me. When the ball is headed forward by a City player, Rodri is offside. Once Mings makes a deliberate attempt to play the ball, Rodri is no longer offside.
This law has come up a few times in the recent past- I remember Vardy scoring one for England where he was in an offside position, but it came off of the head of the Wales defender (I think Ashley Williams?) so the goal stood. Last night's incident looked a bit different but it applies the same law.
Perfectly good goal and, while fans can be forgiven for not knowing about the technicality of the law, Mings certainly should have been aware of the threat Rodri was posing.
Rodri is the kind of man who probably knows the football lawbook like the back of his hand just in case he ever needs to remember technicalities like that. I'm sure there are a lot of players in our squad who wouldn't have realised that they were allowed to do what he did.