I think the lino just got it wrong, they were meant to raise their flag after the phase of play has been completed if they believe the player was offside. I'm not sure if that's still the instructions now but if they flag early before the goal is scored and the ref stops play then mistakes...
VAR gets involved with the Gordon goal because the assistant ref flagged for offside to disallow it. The only person to blame there is the lino for the erroneous flag, VAR did what it's meant to do for once.
Sanchez didn't save either of the penalties against City or Everton. Sterling and Calvert-Lewin both blazed over the bar, they go down as misses rather than saves.