Problem: Editing new page modifies page default

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When I create an Instance of a page type using the Composer, the page type's Default remains unaffected, which is good and expected.
-but-
When I create an Instance of the same page type by selecting "add page" in the sitemap, and then edit it, the page type's Default is also updated with the new content, which is problematic and unexpected.

Why would this be happening?
Any help would be appreciated.

Thank you,
d