I've found that if you turn the key all the way, it binds the seat up. If you only turn it part way, the seat will come off. I think what's probably happening is that the catch in the pin on the actual seat is hitting part of the lock in the bike. Not turning the lock all the way, keeps the moving locking component in the bike out enough to stop the pin on the seat grabbing the lock I think. I'll have a look at mine now and see if I can figure out what's going on.
2017 KTM 1290 Super Duke GT