The mechanics of a die in a press is not rocket science. Variances in OAL are caused by inconsistant projectiles, press flexing, or operator error. The seating die once set is not going to turn in or out on you unless it is not installed and set properly in the first place.
It would still lock the same every time, unless you are chasing an OAL every time you make a round and readjust it(Bad idea). It is not going to move up and down under load at a setting it was left at.It looks to me like the seater plug is being pushed up because the threads are not right.