What would you do?

[LMan]

Agreed, and if you don't at least give the sweeping motion for 'off the bag' you will agin invite dispute as the throw clearly beat the runner.

I would be inclined to a normal 'safe! off the bag!' and let it develop. If the runner acquired the bag, he acquired the bag. He's as safe as if he stepped on the bag until/unless he's appealed.