From the bug notes, I can see that there was no action taken on this item stating that the functionality and architecture for cleaning threads on tab close has changed in latest versions and no longer replicable.
Maybe you can raise an SR to troubleshoot this on 7.1.8 and we can reach out to SME's if there is a product issue.
The HFix-21087 was provided on 6.2SP2 for this behaviour
"submitting a flow action that closes the work object when there is no assignment to perform, users see note data persist on the clipboard for that work object's thread. Subsequent work objects opened into that thread will have that note data saved into their notes." The note in one Wo persists on another, without even being committed.
The RemoveThead activity usage has been almost reduced in latest versions, but you might still see the usage of this activity if you are running older portal on latest platform versions.
I think the behaviour is changed and combined into a single activity DoClose which clears clipboard pages, removing locks, removing thread etc..
You can look into Work- DoClose activity on 7.1.8 if required.