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[Michael]

George,

I don't use gmail and will not going to verify; but anyway. It may be the case that gmail shows different 'virtual' views no matter where they store those mails for real, and never move anything 'for real'. Maybe their API simply does not have such a feature, not internally and also not for remote MUA clients ?
Or maybe you can / need to configure gmail for that feature (e.g. switch off 'all mail'), in addition to claws.
Try searching the web.

What you can do to help tracking it down is launch another MUA like Thunderbird and compare if they can do it. 

Try Thunderbird with https://addons.mozilla.org/en-us/thunderbird/addon/copy-sent-to-current 
and note there are some configuration options, e.g. meantioned here 
https://superuser.com/questions/67488/can-mozilla-thunderbird-show-sent-replies-in-threaded-view/795667
which is arguably some years old now.

John,

> Claws is not a database, its an MUA.

Well at least for POP a MUA does some storage management which could be described as (simple) database. 

But you probably meant that a MUA is rather acting as just a frontend to the server side database when using the IMAP protocol. Still, the question is, who is not compliant here: gmail, claws, or maybe even the IMAP specs are  lacking ?