I know this may be an old question, but I encountered same issue and solved it by creating a content snippet with link to the manifest file in it.

1. Go to Power Pages Management
2. Create Content Snippet. (I would say the only dynamic values here are the website, display name, and HTML content itself; the rest of them are fixed value: Name=Head/Bottom, Type=HTML, Content Snippet Language=English; so that power pages can detect and generate the snippet for head tag of your site).

Hope this is useful for anyone who still has the same issue.

Regards,

Jonas

https://community.powerplatform.com/forums/thread/details/?threadid=7b6c4278-4444-47da-acbc-d3cdd7c7f3c5