Unusual AF Lock behaviour

[alwe]

Before the X-S10 I've had a X-T20 for four years and the following problem is not something I had on that camera.

I'm used to setting a Focus point, pressing the shutter halfway and then compuse my image before pressing the shutter all the way. This always worked on the X-T20. On the X-S10 the Focus sometimes locks, the is a green AF On sign in the upper right corner of the touchscreen and the AF lamp blinks continuously on the back. Changing the framing and pressing the shutter again will not focus when that happens. This has lead to many unsharp images even though I have Focus priority enabled. I can only unlock the Focus by pressing on the green sign on the touchscreen. Then I can focus anew with the shutter button.

I don't know when or why the Focus locks this way and I cannot repeat willingly. How can I stop this behaviour? This is they only thing that annoys me about the X-S10...