Inside a minute left in game, Team A needs to score to regain the lead. They run a pass play to middle of field, pass is caught and receiver tackled immediately but made 1st down yardage. Clock stopped, ball is spotted, U steps away and R winds. As the ball was being spotted A was hurrying to the line but A86 thought he was being replaced so ran off the field. Coach sent him back out and he got o0n the line and set. All of A was set for a second before snap. As ball is being snapped L shuts it down and flags A86 for illegal substitution. Is this correct? It is a dead ball foul?