Perhaps it's the pic but your saddle does not look level, which would cause your symptoms.
Can you check that by making sure tire pressure is the same F&R, rolling it onto a flat floor and placing a carpenter's level across the saddle?

It is normal to slide back & forth on the saddle: Back to push over hills, Up "on the rivet" when powering along TT'ish but you should be able to settle and not really notice the saddle at all.

-Bandera