I just attempted to recreate the issue, but the ‘child page’ location rule is working perfectly for me.
I created 2 field groups to match yours, so perhaps there is something else going on which is causing the issue.
Are you using any code that could in any way interfere or modify the ACF plugin? Hooks / filters?
You mentioned before, a link to another thread and talked briefly about modifying the JS?
Perhaps you could create a new blank site to test the location rules to, and then find the piece of code in your current site which is causing the issue?
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