EDIT

Book in this series:

title: "Taxonomy - biogeography - diversity"
datePublished: "2012"
editor: "Lange-Bertalot, H."
publisher: "A.R.G. Gantner Verlag K.G."
volume: "23"

The resulting titelCache only is "Taxonomy - biogeography - diversity. 23. 2012"

Expected titleCache is: "Lange-Bertalot, H. (ed.): Taxonomy - biogeography - diversity: Iconographia Diatomologica. 23. 2012"
If no author/author team is available AND an editor is include the editor should be cited in the cache. The series title is absolutely necessary.

[@ Andreas M.: this caching problem arose because at Berlin Model times Series with ISBN have been treated as Journals violating a proper referencing]

As described in https://dev.e-taxonomy.eu/redmine/issues/7223#note-19 the print series is meant only for grouping not for formatting. Therefore only the string field series should be used for formatting. Information from the PrintSeries generally does not go into the formatting result. All information needed for the formatting should be within the book itself.
Therefore I suggest to close this ticket as wontfix

Hi Andreas this is your turn. Please see Hennings answer in comment #10

I can only repeat what I wrote in #note-6 . Current understanding of PrintSeries is not that it is part of the titleCache but simply for grouping. At least I think this is how it is used by many projects so we cannot simply change this behavior before having project specific formatters (urgent issue). (otherwise we may destroy others data)

Correct: "Iconographia Diatomologica. 23" is absolutely necessary to identify the book. For my understanding the "print series" field was exactly for this case to get this atomized data into the reference.

True, so for now you should add it to the title of the book.