Referee Szymon Marciniak immediately accepted his assistant’s decision and interrupted the game with a whistle. A few tenths of a second later it took over Matthijs de Ligt took the ball and slammed it supposed 2:2 ins Tor.
Already at that moment it was clear: The Video-Assistant-Referee (VAR) could no longer intervene and overrule the probably wrong offside decision because the game had already been interrupted by a whistle.
The referees in Madrid thus deprived themselves of the opportunity to have the VAR correct a potentially incorrect perception. To prevent this from happening, attacks in close offside situations are usually allowed to proceed by the referees and are only stopped when the actions are finished.
Why this practice was not used in this delicate case can only be speculated upon. Presumably the assistant was sure that Mazraoui was offside. Whether this was the case was decided by UEFA by means of their automatic offside detection technology at least not yet resolved. However, the TV images do not show any offside position.