Where are you carrying your phone? The iPhone is capable of tracking just fine despite what many Garmin owners will tell you, but it needs a decent view of the sky. At the top of a backpack / Camelbak or in the back pocket of a jersey works well.
Been using it in my Jersey back pocket for over 12 months with no issue, had an Android previously and this worked fine also in the rear pocket.
Handlebar mount gives me a “Garmin quality” trace*. Sticking it in your back pocket and trying to see the satellites through your arse won’t.
Have been thinking of trying this but not sure if the reception will keep stable when going down rough technical terrain 😕
I am sure it is a phone issue just not sure what, I was hoping someone had the same problem and found a solution.