Hi There,
Thanks for writing in!
Seems strange that the page template got changed automatically. Can you please try changing the template to No Container | Header and Footer template?

If again it changes to default template automatically let us know your website details so that we can have a look.
Before sending us the details please follow few troubleshooting processes.
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Clear all caches including browser cache then deactivate your caching plugins and other optimization plugins.
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If you’re using a CDN, please clear the CDN’s cache and disable optimization services.
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Test for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
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Remove custom CSS, Javascript and templates.
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Reset your htaccess file by renaming it to .htaccess-bak. Then in WP Admin Menu, go to Settings > Permalinks and just click the Save Changes button.
In case the issue persists, would you mind providing us with login credentials so we can take a closer look? Please provide following information:
Set it as Secure Note
- Link to your site
- WordPress Admin username / password
All the best!