I was running into a very similar situation. I have the Samsung setup with Gear S3 watch and S7 Edge phone. All pretty new stuff, but....

One of the reasons for the watch was the HRM and, on paper at least, it appeared as though I could use it on my rides with Strava or Endomondo (pick any popular app as I think I have tried them all) and NONE would allow me to record my HR from my watch through the app. In fact, most companies acted like I was crazy because I wanted to do so.

Now, let me clarify. I can use my watch if I use the watch app and my HR will record, but the watch apps don't see my speed/cadence sensors. The phone apps see the speed/cadence but not the HR from the watch.

The problem seems to be that the watch apps are not "extensions" of the phone apps. They each record your exercise separately and will not merge the two once the duplicity is recognized...rather you have duplicate records.

The OpenRider phone app is the exception that I found. The watch is more of a remote control for the phone app and it will pull in the HR readings from the watch with a paid account. I was very excited and very disappointed by the execution. I am not one for negative reviews but the app just sucks!

Long story short, now that I have this great gear, I am shopping for a heart rate monitor and I will likely go ahead and but a computer as well save my phone for calls.

BTW I am looking at the Lezyne Macro GPS, any users?