Sorry I don't check here often.
I had my '17 z650 in for service with ABS problems twice now.
First problem, when I initially posted this, ended up being due to a very slight warp in the ABS sensor rotor. It was very consistent to replicate this problem , start the bike ABS check would come on like normal, pull out light goes off, above about 5mph the light would come back on, over 30mph light goes back off... weird stuff..
Side note: my gf has a 2020 z400 and the ABS sensor rotor on that is far better made, its a much thicker piece of metal probably twice as thick as my 17 z650
Second problem happened November 2020. This time it was much more difficult to replicate. Sometimes ABS would work fine, then while riding (consistent speed, not rough road) the ABS light would randomly come on. If you turned the bike off / on sometimes it would clear the ABS check light, then randomly while riding it would come on again. The dealer initially told me they found no problems I asked to try again and the ABS check light did come on while riding. This time they replaced the ABS hydraulic unit (again)
I rode it home and it seemed fine, I then put it away for the winter. I am really really hoping I don't have any more issues come this spring after I take it out of hibernation.