@MatteoB729Your latest reponse indicates it may have something to do with your S9. Now try it on another S9. If it is the same problem - it is a generic issue. If not, it is something specific to your particular S9. In the first case, it may have something to do with the version of your Android operating system or it could be an outstanding incompatibility which could reside in either the S9 firmware, the Android operating system code, or the QC35ii firmware and the way it interacts with an S9. In the second case, you may be able to update something on your S9 to make the issue go away.
Check out the other posts here again as over the last few months I have seen other S9 users having issues with the QC35ii. The frustrating thing will be unfortunately, if you can't update or change something on your phone, you will need to depend on one or more of Samsung, Google and/or Bose to make changes to their stuff and in the best of situations that could be slow to materialize.
There could very well be an issue with the S9 (and the up and coming S10) and that is why I previously made the troubleshooting suggestions I did. However, not working for Bose, I personally doubt that per se BT 5 has anything to do with it as the protocol is backwards compatible with BT 4.x. If there were a global issue with BT 5 devices, then we would also see the same issues coming up with the iPhones 8 and X which also support BT 5 (did I miss something here about the iphones? 🙂 ).
Suggest you troubleshoot as I suggested or otherwise to eliminate all the other possibilities including your actual device to determine if there is a 'generic' issue with the all S9s.
@MatteoB729I don't know the S9 so I can't tell you if changing settings will make a difference. You want to methodically test to determine if it is an issue with (a) your Internet connection, (b) your particular S9 or (c) all S9s. Right now it is not yet obvious where the 'issue' resides. From the other posts here, I would lean towards an issue with all S9s but you have to test and determine that for yourself. If that is indeed the case, you may or may not have a 'workaround' and would have to wait for Samsung, or Google, and/or Bose to make a change. Remember not just to check out a local audio player, but play an audio track that is local to/on your phone. You want to know if it is a 'streaming' issue or a audio playing issue.