Why does VoiceOver iOS revert the cursor position after tabbing away from an input field?

While testing VoiceOver for iOS within an @ionic/angular cordova app, I encountered the following issue:

I have two elements on a page. One is an h1 with a tabindex of 0. The other is a plain <input>.

1. I enter into 'edit mode' on the input field (double tap with VoiceOver on)
2. I reposition the screen reader focus to the h1
3. I double tap the h1

I would expect at this point that the screen reader focus would remain on the h1. Instead, the focus shifts back to the <input>.

This can be reproduced by opening this StackBlitz in Safari on iOS and following the above steps.

Notes:

• the tabindex of 0 on the h1 is to facilitate focusing at the top of the page after a transition.
• this happens on non h1 elements as well, this is just an example; it seems more related to the tabindex than anything