Log4OM

Perhaps you would care to post the return ADIF here as a zip file so that we can see exactly what is being imported?

Find attached a quick test sample.
1) marked QSO with QSL_RCVD = Y and QSL_SENT=Q
2) export to adif
3) set QSL_SENT = Y in 3rd-party-software
4) export to adif in 3rd-party-software
5) merge-adif in Log4OM
=> Result: QSL_RCVD = N in database

I believe it is because in the return ADIF there is not a QSL received field so without a field we would probably assume that the QSL was marked N

Alternatively there are a lot of <CRLF> annotations and these may be distorting the import because they are not normal ADIF fields.

Have you asked the author of the other program why he does not export the imported fields as received instead he is sending nothing and also why he is using <CRLF> inside an ADIF field.

Also why are you exporting as an ADIF 2xxx format instead of ADIF 3xxx?