7zip Hex — Editor

: Automatically highlight and label known archive structures (like the 7z signature 37 7A BC AF 27 1C ) to help users identify where file metadata ends and content begins.

You associated the file extension with the hex editor, not 7-Zip. Go back to 7-Zip -> Tools -> Options -> Editor. You must set the path to the .exe directly. Do not rely on Windows file associations. 7zip hex editor

If you absolutely refuse to use an external editor, you have two alternatives, though neither is as flexible as the 7-Zip/HxD combo. : Automatically highlight and label known archive structures