One striker stays offside after the attack breaks down

The striker does not need to recover all the way towards the ball, but they do need to reset into a position where they can become involved again. A simple reference is: can you see the second-last defender and be ready for our next pass? After possession is lost, the striker can also help by blocking a forward pass or applying pressure from the front. That keeps them connected without asking them to chase into midfield every time. If they remain beyond the defensive line, they may stretch the opposition, but they are also unavailable if the team regains the ball and a teammate plays forward. Use transition games where the striker has to become available again after a regain. At U12, I would coach the reset rather than the distance they must run. The useful position depends on where the ball, defenders and teammates are. It can be central or wide. The reference is whether the striker can see the line, affect the next pass and become available if possession changes. If the ball is on one side, recovering into that side's passing lane may be more useful than returning to the middle by habit.