Good find JackB1.
Can most of us agree point-by-point to these? --
That having the racket edge-on toward the impact point just after the arm is up & straight is a common characteristic of the current strongest servers. We can check this point in high speed videos of pros.
Having the racket face facing the sky after the arm is up & straight is not
a characteristic of current strong servers. We can check this point in high speed videos.
3) Not WTE.
Some combination of grip and joint motions might have the racket face pointing toward the sky prior to
the arm's being up & straight for the strongest servers (as JackB1 proved). This point could be checked farther in high speed videos. But JackB1's frame looks good and we might assume that we could find more examples, some maybe with timing variation on when the racket face faces the sky.
4) WTE. We might agree that with the arm up & straight and a racket face facing the sky that - at that point - we have Waiter's Tray Error.
5) Possible WTE.
The grips and sequence of joint motions that occur at times before
the arm is up & straight are not so obvious and we need more research and description for WTE to become clear and unambiguous.
6) WTE Cause?
The most important and interesting thing is the murky sequence of joint motions preceding the time when the arm is up & straight..................