Unable to connect iOS 11 iPhone with Samsung watch

When I bought my Samsung Gear S3 Watch, it properly connected to my iOS 10 iPhone.

Upgrade to iOS 11 went fine, including for connecting the watch, however few weeks later I had to reinit my phone.

Since then, the bluetooth of my phone will not connect to my watch (it says Connected, and 2 secs later Not connected). I have the same issue with all my iOS 11 devices (iPhone 6, iPhone 6S Plus, iPad Pro 2017). Forgetting the connecting does not help neither a hard reset of both phone and watch.

Did anything change in iOS 11 re. Bluetooth management or do other people experience the same issue or, to the opposite, success ?

Thanks.