"Merge All Windows" function in Safari is not working anymore

Hello Rgreenough,

The issue could be with having different types of windows open in Safari. If so, what you're experiencing may be expected based on this information:

Merge an app’s windows into one tabbed window: In the app, choose Window > Merge All Windows. If an app has more than one type of window (such as Mail with the viewer window and the new message window), only the active type is merged.

Learn more here: Manage app windows on Mac - Apple Support

You also may want to take a closer look at what you have selected in Safari > Settings, then click General:

New windows open with

Choose what a new Safari window contains:

- Start Page: See websites you’ve marked as Favorites, a Privacy Report summary, your Reading List, and webpages open on other Apple devices where you’re signed in with the same Apple ID.

- Homepage: The homepage you set in the Homepage field below.

- Empty Page: An empty Safari window with no webpage address or content.

- Same Page: The webpage you most recently viewed.

- Tabs for Favorites: All the bookmarks in Favorites as a set of tabs.

- Choose tabs folder: The bookmarks in the bookmarks folder you choose.

You'll find this information and more details here: Change General settings in Safari on Mac - Apple Support

If the information above doesn't seem to help and you still think this isn't normal behavior with Safari, we'd suggest reaching out to Apple Support directly so they can take a closer look and provide some additional guidance. This link will help you get started: Contact - Official Apple Support

Take care.