As it turned out, I had overlooked/forgotten a "995n"-tag in the General panel, which, according to the guidebook, overrides the Source-tab, and this was responsible for the entry "Archaeologia Austriaca, 102 (2018)" in the Journal field. Without this tag in the General panel, the Journal field remains empty, regardless of what I try.
Even the most simple solution
It's possible that the source parser doesn't work with MARC records (which are treated differently from any other source). This code was written many years ago -- I tried it, and like you the tag wasn't imported.
Related to this format: Is there a way to get rid of ending full stops (".")? At the moment, each line ends with one, and this is imported into BE. I know that I could remove all incidences with the command "Remove these characters from all fields" but this would also delete full stops I do not want to remove.
There is a global change -> find/replace, but that also wouldn't distinguish between periods in the title or at the end. It might be possible to do a Regex search for references whose titles have only one period, which occurs at t the end, and then do the global change on those. I'm not enough of an expert about Regex to design that search, but maybe you are (or someone else reading this can).