Could you please let us know the exact version of Lightroom you are using now? Also, do you use Ctrl+z in develop module while editing images?
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You may have the Any Source plugin installed and its Sync Smart Collections enabled. When syncing is enabled, entries named "Sync" appear in the undo history. Unfortunately, LR automatically adds undo entries whenever a plugin modifies the catalog, and there is no way to prevent that.
If you're not actually using Any Source's syncing of smart collections, you can disable it by doing File > Plug-in Extras > Sync Smart Collections and unchecking the option Sync The Selected Smart Collections.
Thank you both for helping and apologies for the delay in responding. Not enough hours in the day - and I'm retired!!
Yes - you are absolutely correct - the culprit was Any Source.