Works here, also. Safari 16.5.1, OS 11.7.8 (Big Sur). Only two extensions: 1Password and URLM Pro (Add Bookmark).
The "Not Secure" goes away if you click in the location field. It shows (for one reason) when the site has not updated to httpS. [Pictures at Ten!] You can copy the text in the location field and paste it into most any window that will accept a text paste and see that the site is still using "http". Other browsers use different alerts/warnings.
Do you have "Adult Only" enabled?! 😱🤭