I am destroying Kor'Tahz race, when I noticed the strange issue: while I definitely blocking two enemy systems, on the "Report" screen in diplomacy it's only one!
Then I found that it's common issue with the every race: Report screen shows only "dedicated" (colored) systems. If system colonized by the several races at once, it must be shown by every one - instead it not shown in the report of any.
Looks like the bug for me.