I stumbled upon this thread as I had exactly the same issue with asus z170i MB and i7 7700k. From what I've gathered after googling the problem for hours, is that it appears to be a BIOS issue. There's a YouTube clip of a guy overclocking his 7700k with an asus z170i using BIOS version 3007.
Unfortunately this version of BIOS is no longer available - someone reckons it was taken down from the asus site for some reason. I missed it and could only get BIOS 3016, that is the latest (and only, I think) version that supports gen 7 Intel. This version does not support ratio o/c as both you and I have discovered. It is also reported as such in hwinfo64.
I have emailed asus querying this. It's obviously largely irrelevant to you as you've moved on 🙂. I was thinking of doing the same thing but may hang onto my cash a bit longer and see if asus come up with a solution to this. I also thought it may help anyone else in the same predicament as it took my hours to track the issue down 😐