Keen to know of any updates on this issue. Having the same problem (Galaxy S8 and Gear S2). Have tried the permissions and force stop but that only worked for a short time after that the stopped syncing the phone battery percentage again.
Are you using a free face or a premium face? This isn't a perfect fix but if you are using a premium "themeable" face you can force a connection between the app and your face by changing the themed color within the app. This isn't ideal but it's a quick fix for the time being.
It's a premium face. But one of the things I do to try to get phone percentage back is change the watchface, usually to one that has battery percentage and does work. Then change it back to facer later. Sometimes it works but most of the time not.