If you uninstall samsung pay app from your phone, smart lock will work after reboot. Samsung pay seems to disable smart lock for added security. If you use samsung pay you will have to live with the intermitten problems with smart lock. It's only pay app you need to uninstall.
I don't have Samsung Pay installed, and my Smart Lock does not work, AND I have just received it back from Samsung repairs... they have not fixed it at all, same problem, just wasted my time. I will send it in again. I have a spare phone, I will keep sending it until they fix the problem...
I've managed to resolve the problem on my S8. It ended up being that I had more that one device using my Google account, and all of them updated the location. I'm sharing this link. Hopefully it can help someone else as it helped me.