Non-existent URLs Showing Up With 200 HTTP Response

I am not sure why we’re suddenly seeing non-existent URLs of our site on search results, such as: http://www.criminal-lawyers.com.au/tag/sexual-offending-allegations-of-sexual-assault and http://www.criminal-lawyers.com.au/tag/royal-commission.

Although they are displaying our intended 404 page designed to advise our readers that the page does not exist, these pages however are returning 200 OK HTTP response codes.

Can anybody guide me on what’s causing these non-existent URLs to show up and how do I fix the wrong HTTP response for these pages?

Note: Other non-existent pages are correctly returning a 404-response with the 404 page. The problem seems to be with pages with “tag” on the URL.

Any help will be greatly appreciated.