Camera.computed.target_info is none during PostUpdate only on mobile

## Bevy version and features

Bevy 0.16.1

## What you did

I got an Issue on [bevy_ui_anchor](https://github.com/TotalKrill/bevy_ui_anchor/issues/15) where a user noticed that during 'PostUpdate' the 'camera.computed.target_info' was always 'None' during PostUpdate. This was only the case on iOS and Android

The user then moved the system into 'Update' and then it started working again. I went through the documentation of the said fields and could find any reference to why this would be the case.

## Expectations

The value should exist or not exist on all platforms, or the documentation should somewhere highlight the reason for this not being the case.

The report names no source file or test. Begin by comparing camera.computed.target_info during Update and PostUpdate on iOS and Android, using the linked bevy_ui_anchor report as reproduction context. Done means the value behaves consistently across platforms, or the relevant platform or scheduling limitation is documented.

Written by the indexing model from the issue text.