I have had the same problem with the 18-55 which I just sold. I have just picked up the Samyang 85mm from Korea and it has the same amount of movement. its the mount itself that is the problem. My prime 35 has minimal movement which is perfect as my canon primes had similar amount of leeway. I'm going to travel into a local store and check the other X-T1 bodies and my lens + other Samyang lenses to see if they are tight enough.
I'm more worried about this issue damaging the lens mount itself seeing that it rocks back and forth