If I use a former used writer sheet with fields and a database on basis of a calc tabel and I switch to another calc tabel as new database and also change all fields to this one - it will constantly send the mails on basis of the old calc tabel. It makes no difference if I also delete the old database off the registered ones. I had to create a new writer sheet to get the new database going.
@Andreas: please provide us with: - a test Writer document containing the fields for mailmerge; - the first Calc file from which the fields are inserted into the Writer document; - a second Calc file from which the replacement fields are to be pointed to in the Writer document. Please also explain step-by-step how you exchanged the first datasource for the second datasource. Please also indicate your Linux OS and version, and which LibreOffice packages you are using, e.g. distrib-provided packages, or other source (e.g Flatpak, Snap, etc)
Your report might be a DUPLICATE of bug 107896
@Andreas: 7.4.x is end of life since 12th June 2023 (unless your distrib is providing). Please try with the most current release, given the report in bug 163700, that might not make any difference.