Scoring from an off-side position

[Charlie]

I may not understand this, as no pundit etc are raising the issue. However, I am wondering how Casemiro's goal against Nottingham Forest is allowed as he was in an off-side position after diving/falling over when the players ran to goal expecting the free kick to go towards goal, when it didn't all defenders ran out leaving him on the ground. The ball is played in and headed down to Casemiro who then scores, no N Forest player touches the ball or gets as far forward as Casemiro.

2 questions; How is he not off-side? How is this a goal?

I have followed football for almost 50years and understand the concept of off-side, but need help to comprehend this one.

Charlie