Thank you for your hints! I didn't want to write to Fuji Germany (in German) because I figured that they would forward the request to Japan anyway and then half of it will be lost in translation.
I got the answers I was looking for at DPR. Basically:
- the IBIS' gyros are always on because the sensor is floating. Even when IBIS itself is off (in which case the gyros keep the sensor at a static position)
- Fuji's Auto-ISO feature has weird limitations and we can only hope for Fuji to fix them in the future