I am having the exact same issue with my s8 and Gear S3 watch, ICONX-2018. I talked to Samsung support and they confirmed the latest OREO updated has caused this issue but no schedule to fix it.
They suggested I go to Bestbuy and have the flash my software to previous version. OUCH>>>>>>>
or try to do factory reset of phone and watch, this works for me but it takes lot of time and to do backup of all your settings/apps... and to do this every single time there is new version of gear app or gear software... quite annoying
I'm doubting that in my case problem lies in some security conflicts as I have also company account on phone...