I think it must be optical trickery.All I can say is that images doesn't match what the video shows. Could just be optical illusion but in the video the player is well onside.
Here is the video:
Here is the In line:
And again the reverse angle:
All taken at around the same point, close enough that the positioning doesn't change.
We have to trust the tech here imo. The offside player is the next person to play the ball following the touch from the player challenging GK which is the only questionable part of it for me, although I think the way the ball goes up in the air is consistent with glove to ball to Ecuador head