I seem to be having the same problem as SimonW in his post from April 4, but that thread didn't seem to reach a conclusion. I've been using an X-T1 since 2014 and enjoy it very much. Two weeks ago I purchased an X-T5 and I've been mired in camera menus and owner's manuals and message forums ever since. On the X-T5, the "Set Up menu > Button/Dial setting > AE/AF Lock Mode > S" should produce an ON/OFF switch for the AF ON button and the AEL button. This is a simple and elegant solution for Back Button focusing where you want to hold focus at a fixed distance with changing backgrounds or changing subjects at the same distance. One simple button push and you are back to focusing by half-press on the shutter button as your needs change. Having to continuously press the button to maintain focus makes my grip on the camera less secure, and subjects my focus to errors when my thumb slips or moves even slightly.
That's exactly the way it works for my X-T1 and the exposure lock on my X-T5, but not for the autofocus lock. Autofocus Lock only stays "locked" while the button is being pressed on the X-T5.
Does anyone with an X-T5 have the menu set to "AE/AF Lock Mode > S" and have both focus and exposure locked until the button is pressed again?