There is no point to keep Bitdefender Internet Security along with default Windows Defender, which may cause conflict. In your case It the offender is Bitdefender who makes it always not hidden as a more security level.
Hide File name extensions
I have Windows 11 and my antivirus program is Bitdefender Internet Security. I want to hide File name extensions. I clicked on the View button in File Explorer, took the cursor over Show and unchecked File name extensions. Nothing changed. I clicked on the three dots near View, clicked on Options, then on View and checked Hide extensions for known file types. Then I clicked on Apply followed by OK. Nothing changed.
Someone suggested that I should uninstall and re-install Bitdefender Internet Security, but I haven’t done that.
File name extensions have become visible only 7-8 days ago.