SPYROS P.

Well, i had the same problem with Fuji XT50. The solution is: first you set one of the auto options of ISO the low, and the high iso. Let's say: Low ISO in 125 or 200, high ISO 1600 for a sunny day. Or higher in a dark day: 6400. Then you must go to set up menu (Symbol a wrench) choose button dial setting and the you set LOCK. Then in LOCK choose / SELECTED FUNCTION and then / SELECTED. After that yoy go below, you choose FUNCTION SELECTION to block unwanted activations. In your case, you block ISO SETTINGS. Then when you press the Q button you will see the iso mode DEACTIVATED. By the way, this happens because you use shutter speed priority mode and the camera detects incorrect (usually very low speed. The problem is that the camera doesn't return to the previews selection and remains in high iso settings. Good Luck! (Advice: NEVER de-activate the LCD screen for reviewing pictures!)