This is because when viarewind goes to move the hologram, it removes the wither skull from the top of the horse, and then puts it back on after teleporting both, which is not necessary, when you only teleport the horse (the client will automatically teleport the skull) avoiding the flick.
Currently there is an issue with the hologram flickering as it moves:
https://streamable.com/r23qf0
This is because when viarewind goes to move the hologram, it removes the wither skull from the top of the horse, and then puts it back on after teleporting both, which is not necessary, when you only teleport the horse (the client will automatically teleport the skull) avoiding the flick.
After fix:
https://streamable.com/npefsq