Google Maps Static API - Referer Policy Issue

Question

I have created an API Key, assigned Static Map api only and applied referer policy e.g. www.example.com.

My expectations was this api key should only succeed if http referer header equals to "www.example.com".

When I tried, it's working as below:

1. Successful response when referer header is "www.example.com"
2. Unsuccessful response when referer header is not blank and some other value
3. Successful response when NO referrer header is provided.

Point 1 and 2 is fine however I point 3 shouldn't have been successful. Am I missing something? Is this how referer policy implemented for maps? I tried by creating new keys as well but same results.

Thanks in advance.