Secondary sorting: not working for some special fields

[mlep]

JabRef 5.2--2020-10-13--fdaf25a
Linux 4.9.0-13-amd64 amd64
Java 15

• Mandatory: I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

In the entry table, entries can be sorted by clicking on the column title (primary sorting). By holding the SHIFT key, a second sorting criteria can be used.
It works with a primary sorting and a secondary sorting on fields like "Year", "Title", "Author", etc.
But if the primary sorting is done on the special field "Relevancy", "Quality" or "Print status", the marked entries are not sorted based on the second sorting criteria. Only the entries that are not marked are sorted.

Steps to reproduce the behavior:

1. open the file jabref-authors.bib (https://github.com/JabRef/jabref/blob/master/src/test/resources/testbib/jabref-authors.bib)
2. display the columns of the specials fields (options -> preferences, tab Entry table)
3. mark a small set of entries with "Relevancy", "Quality" and "Print status"
4. sort entries by "Relevancy" (or "Quality" or "Print status") by clicking on the column title, so that the marked entries are at the top of the entry table
5. hold SHIFT and click on the "Year" column title (or "Author", or "Title", etc.)
   ==> Only the entries that are not marked with the primary sorting criteria are ordered according to the second sorting criteria.

Note: it works for special fields "Read status", "Rank", and "Priority".

[mlep]

@Fu188 : Thank you for fixing this issue and making JabRef a better software. Now, it works as expected!

JabRef 5.3--2021-02-01--66a771c
Linux 4.19.0-14-amd64 amd64
Java 15.0.2
JavaFX 15.0.1+1