Summary

There appears to be a reproducible issue with the sub-page block when editing a page in the CMS.

When the page first opens in edit mode, the sub-page block appears to be missing or blank, even though it is still present. Attempting to preview the page while this block is present also causes the preview to load indefinitely.

This is particularly confusing for general users and teachers, as it can appear that the block or its content has been deleted.

Steps to reproduce
Open a page containing a sub-page block in edit mode.
When the page initially loads, the sub-page block appears blank.
Double-click the blank area where the block should be.
Make no changes and click OK.
The sub-page block will then appear correctly in the editor.
Double-click the block again.
Remove any one of the listed sub-pages and click OK.
The selected sub-page will disappear from the block in the editor.
Attempt to preview the page before saving the draft.
The preview will continue loading but will never display.
Cancel the preview.
The CMS will then remain on a loading screen indefinitely.
To recover, navigate away from the page and reopen it.
Expected behaviour

The sub-page block should display correctly when the page first opens in edit mode.

After making changes, users should be able to preview the page before saving so they can confirm that the changes look correct.

Actual behaviour

The block initially appears blank, despite still being present.

After editing the block, the preview does not load and leaves the CMS stuck on a permanent loading screen.

Additional information

If the sub-page block is removed entirely, the page preview works correctly. The issue therefore appears to be specifically related to the sub-page block.

The problem is consistently reproducible and could cause users to believe that the block is missing or that content has been deleted.