Activity Pane greyed out in shared Numbers file

I want to check the activity panel for recent edits to a shared Numbers file (of which I am the owner, shared to about 6 other accounts/devices).

When viewing from my Macbook (macOS Sequoia 15.3.2), the View>Show Activity Pane option is greyed out, and when viewing from iPad or iPhone (both iOS 18.5) the Collaboration button doesn't appear in the toolbar, so I cannot click on Show All Activity as suggested in the Apple Support article.

(See the latest activity in a shared spreadsheet in Numbers on iPhone - Apple Support (IN))

There are no sidebars open, and as far as I can tell I am not in any particular mode that can be exited from as suggested in the above article.

On icloud.com web view, clicking the Collaboration button does not bring up any further dialog boxes or windows.

If I am logged into the file from two different accounts simultaneously, I cannot see the cursor of where the other account is viewing (as I can for other shared files).

I tried changing the file type from Package to Single file but that seems to have made no difference.

Thanks for any suggestions.