WAL

Looks like motion blur since it is unlikely the roof of the building is moving the vibration is originating from your setup! Obviously, the sensor resolution will reveal more imprecision (vs full frame) and your A-B test seems to verify motion blur when using slower ss. If you have not already, try shooting with AF-S mode, timer on (at least 2secs) or remote w/hands off rig and both IBIS and OIS off. If that doesn't work try electronic shutter with aforementioned settings if you have not already. I have not yet shot with your 250 lens. However, on the GFX 100s I have shot with my GF 45-100 and using a lens adapter with 70-200 Nikon lens on a sturdy Gitzo carbon tripod and a head that can hold 600mm lens weight. So far, I have done long exposures (in the minutes ss) with both IBIS and OIS off when shooting with a longer focal length and have not encountered this issue. Good Luck!

Have used FB on the GFX 100s in both MANUAL AND AUTO modes and have not experienced your issue. My only complaint is AUTO mode tends to shoot more brackets than necessary. But that's not your issue. Have you looked here? FOCUS BKT is the 4th bracket down. https://fujifilm-dsc.com/en/manual/gfx100s/taking_photo/bracketing/ What are your settings for these? (CAMERA MENU) SHOOTING SETTING > FOCUS BKT SETTING - MANUAL or AUTO bracketing? MANUAL: FRAMES / STEP / INTERVAL? If AUTO FB then did you go through the procedure in the manual? What AF mode are in?