This is just how Fuji AF algo works all throughout its cameras.If your background has better contrast (vertical lines) than foreground, you really have to be sure your AF point is completely inside your subject, not even touching the background. It is actually almost impossible to do with OVF as AF point there is huge. I am also getting a lot of pictures in the parks with focus on background but I check every picture at 100% magnification and delete misfocused pictures (you have to shoot RAW+JPEG as you won't be able to magnify your RAW 100% or even 50% due to Fuji implementation). There is not hing wrong with your camera, this is just how Fuji engineers programmed it.
While researching other systems, I tried Sony A6300 and it is also doing that from time to time in S-AF mode (I would call it occasionally, not systematically as Fuji) but its C-AF mode with face and eye tracking is so much better and useful. I tried to use C-AF mode in my X-PRO2 instead of S-AF and in many cases AF precision improved. Although C-AF has a couple of major flaws in all Fuji cameras, one is that exposure is always locked when you half pressed the shutter button and another is that it also like background. Even Fuji's face detection mode is focusing on background in a lot of cases (it says "face detected" but it is completely out of focus)
Micro 4/3rd cameras almost never do that but I found a couple of photos from my old Olympus camera collection with focus on background instead of subject too but it is less noticeable due to a larger DOF at the same aperture values.
I did not find any misfocused (on background) pictures with my old Nikon D750 but in general its AF precision was worse than Fuji even in good light, that's why I am shooting with Fuji.
This is just something you have to adjust to if you like Fuji cameras and lenses hoping that they improve the AF algo in the future firmware updates. Alas there are no good alternatives. Nikon D500 AF is amazing but Nikon primes are pretty soft wide open and camera is huge. Sony a6300 has amazing C-AF but pretty bad S-AF (especially in low light). I would go with A6300 if it had focusing joystick (the way it is done in D500 or X-PRO/X-T2) as using S-AF with Sony is just a pain. Micro 4/3rd are amazing cameras and even better lenses but they are using outdated sensors in their best cameras (Olympus E-M5 II and Panasonic GX85). I wish they added good OLED viewfinde, new 20Mp sensor and PDAF capability to GX85. But they deliberately crippled it with older sensor and mediocre viewfinder just to keep cost lower and not compete with their other cameras.
I tend to think that I will be replacing my X-PRO2 with upcoming X-T2 just because it will be easier to see the auto-focus errors on its bigger viewfinder. It is annoying that I have to check every picture but the ones that I get with right focus are outstanding.
