It's a bit of a bug.
The idea is that if you edit a page called _Footer it appears as a footer on every page.
Your page is called OUT_HTML_FOOTER, and I think the system also thinks that is a match for the _Footer page.
I think the best thing to do now is report this to GitHub support.
Thanks for this feedback! I've logged this bug. Though I can't share a timeline for this, I can tell you that it's been shared with the appropriate teams.
Please let me know if you have any other questions.
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