Why was Germany’s extra-time goal against Paraguay disallowed after a VAR review?

Germany's Jonathan Tah believed he had scored a headed goal during extra time against Paraguay, only for the effort to be disallowed following a VAR review. The Video Assistant Referee intervened to overturn the on-field decision, denying Germany what would have been a lead in the match. The specific infringement that led to the disallowance was the subject of scrutiny following the review.