I don't believe that Zarha's Lash actually does anything to a mounted model.
The key part of the Lash rule is "when resolving push effects..."
From page 14 of the rulebook, dealing with mounted models:
"If an attack targeting the rider successfully pushes the rider, the rider is forcibly dismounted. If this happens, place the rider in base contact with the mount, directly opposite the attacker, then resolve the push."
So the dismounted rider is already placed behind the mount before Lash triggers.
Agreed. The push resolves after the rider is dismounted, then is pushed (towards Zahra) but hits a larger base and stops.
My opponent understood lash to supersede the rules, but of course, I didn't want that happen.
Link to my Arena Rex Albumhttps://photos.app.goo.gl/Ljkv5txvhzVcSt529