It was backported to 7-3 branch before 7-3-3 was branched, so it appears in all 7-3-3 point releases.
The “exactly same” is “it doesn’t work”, with everything else different: different OS, different results (flashing window and continued “normal” operation vs crash/hang).
I really suggest you to try 7-4 daily in parallel from Index of /daily/master/. I will likely be unable to debug and solve a problem that doesn’t appear on development branch, or is Linux-specific.
So I upgraded to 7-3-4 (the latest version available on openSUSE Tumbleweed) but the issue was still present there. However, with the 7.4.0.0.alpha1_2022-06-10 daily, MailMerge is working as expected and email is being sent.
Thanks a lot for the suggestion! I hope the fix can be backported or alternatively that 7-4 will be released soon. In the meantime, this is at least a workaround allowing me to work.
I downloaded the daily here: Release 7.4.0.0.alpha1_2022-06-10 · clin1234/libreoffice-appimage · GitHub