How do I stop Safari from deleting my tabs on iMac?

Yesterday 18 tabs on one window and 9 tabs on another window disappeared upon re-opening Safari on my M1 iMac. I managed to restore all tabs from the history and then they all disappeared upon the next re-opening of Safari - just displaying the start page. Upon which they were not restorable from the History.

This is infuriating as I have manually opened all of the tabs, from memory, again, to then lose them all upon next opening of Safari

This problem first occurred on 8 April 2024 - I was not aware of any software updates from Apple (other than Pages, Numbers, etc) or any other specific changes to my installed software, app, hardware, etc. At the same time I lost 7 tabs from three Finder windows

I have tried all the usual checks - open windows on restart is always checked, iCloud is configured, re-booted, Utilities first aid, latest MacOS is up to date, etc

My other devices seem to be normal

Pinned tabs are still showing - perhaps I should pin all new tabs to ensure they remain ...... (although this will be a pain too)

Any suggestions will be most gratefully and desperately received

[Re-Titled by Moderator]