ISSUE RESOLVED!

The zoom issue was caused by a simple focus call (input.focus()) to an input element upon loading the page. Remove the automatic focus - problem gone!

Manually clicking on the input to get focus after the page loads does not automatically zoom the page again.

Nevertheless, I still cannot understand why it zooms into the input box with extreme magnification. I could only assume that the zoom is calculated on the original dimensions of the page instead of the transform/scaled dimensions.

While I had originally noticed this behavior on the iPhone 17 Pro, I can now confirm that it occurs on some earlier 14/15 models as well.