I suspect you first look through the telescope and focus on the subject without the iPhone being involved. In this case, the telescope focal point is fixed on your eye. with a very short distance between the telescope eyepiece and your eye. Being such a short distance probably means the focal point position is very sensitive to movement.
The iPhone will adjust its lenses to focus the iPhone sensor. But with such a short distance from the phone to the eyepiece, the phone might need to continually adjust if it moves at all. You might need some attachment to hold the phone steady.
It might be that any movement of the iPhone might cause the iPhone to continually adjust to properly focus
Then when using the iPhone, it will try to adjust its own lenses so that the image is focused on its internal sensor.
It might be that slight movement of the iPhone might cause the problem.
I think there are multiple focus modes in iOS that are different between phone models, including ways to cause a fixed focus. Do a search for different focus capabilities that apply to your model and version of iOS.
Finally, I understand that some iPhone models have a LiDAR scanner for measuring distances. Perhaps that scanner receives signal bouncing back off the telescope eyepiece.
I'm guessing most of this, but I hope it helps a little.
