Subpages are not copied for templates served by a provider with "Save and edit" action on create

How to reproduce:

• Create a template page, that is a WebHome (a non-terminal page)
• Create a WebPreferences corresponding to this WebHome page
• Configure a template provider to serve this template
• Set its "Action on create" to "Save and edit"

• Create a new page from this template (by choosing its provider in the create page screen)

Expected result:

• A new nested page is created, with 2 pages: WebHome and WebPreferences

Actual result:

• The page is created but only with the WebHome page

If the "Action on create" is set to "Edit", all works as expected.

This is particularly annoying for templates whose WebHome pages contain a dashboard, since the dashboard does not work correctly if action on create is not Save and Edit (since it's loaded async from the server, and if nothing is saved, nothing exists outside of the current request).