Sorting by any column

I have the following columns in my Jabref library:
entrytype, author/editor, title, year, journal/booktitle/bibtexkey, times-cited,review

and I want to sort the entries according to any of them. When I sort by entrytype or author/editor it works fine and the order is correct. But if I sort by any other column (e.g. title as in attached file) I get

In this case, there are ‘two lists’, but if I sort, e.g., by times-cited I get more of them. At first I thought it is related to groups I have, but I cannot find any justification for those two lists when sorting by title. Of course I selected ‘All entries’.

What am I doing wrong?

JabRef version JabRef 3.6
windows 7 6.1 amd64
Java 1.8.0_102

You are right. I can reproduce the same behavior.

I have filed a bug report at