I have a Samsung S21 Ultra 5g and a Fenix 5X Plus.
Previously I had the same watch and a Huawei P30 pro.
The watch worked fine with the Huawei.
The watch is next to useless with the Samsung.
There is a huge risk when the watch is out of range of the phone that it disconnects and will.never reconnect again.
At this point, when you open connect, there is a + sign where just a minute ago the image of the Fenix 5x plus was.
You have to re-register the device. This can happen as much as 15 times a day.
I'm not sure whether Garmin or Samsung is to blame but it has been an issue for 4 months and two Samsung updates and one Fenix update.
Allied to this, the app doesn't even have to close. You can lose Bluetooth with the app still running, and then permanently lose the watch until you register the device for the umpteenth time that day.
Please resolve this. It really is terrible.