I recently had to reset my Fenix 8 and remove it from the Connect app.
Afterward I tried to re-add the F8 to Connect but it could not find the F8.
I noticed that in the iPhone BT settings that the F8 showed up as connected which I believe is the reason Connect could not find it. Unfortunately, the F8 BT entry had no (i) icon so there was no way to "Forget" the device. I believe that is because it is a BLE connection that Connect did not correctly remove from the iPhone.
After trying several things like uninstalling Connect, restart both the F8 and iPhone, the connection would not go away.
Finally, after spending an hour with Apple support, they suggested resetting all settings on the iPhone and that worked.
However, now I need to redo the settings which is a pain.