I feel sure that I learned, decades ago, at school that the ball is dead "when the umpire deems it to be dead".
In other words the umpires at the Lords' test (including the third umpire), had every discretion to rule Bairstow "not out". He had, after all, been in his ground at the time the ball was taken by the wicket-keeper, and for reasons unconnected with play had left the crease.
The umpires' decision to give Bairstow out seems to me in total contradiction to the spirit of the laws of the game. Was it?