We are encountering this exact same issue too.
What happens if you add a 5-stars rating for a product without ratings manually? Have you tried doing it?
Unfortunately, theres currently no way to manually add star rating on wp-admin.
Please check this github link:
https://github.com/woocommerce/woocommerce/issues/4072
How do you think we can fix the display of the star rating?
Thanks!