Lucas Chess mark "unknown" instead of "Mate" for some games?!

Hi

At first, I apologize, if my question is not belong to this thread, but I didn't see a section for chess programs. I've installed Lucas Chess, and I like it. Sometimes I see this chess program, mark "Unknown" instead of the "Mate" word under "Tools > Database > Autosaves file. In the table under the "Games" tab, you can see all information about games. Some games I lost and the chess engines won, shows "Mate" under "Termination" column (this is correct), while I lose some games and chess engines won those games, mark as "Unknown"?!
I don't know why this happen; while chess engines won all of the games and I expect to see "Mate" in the "Termination" column in the table. Whenever this happens; I have to edit game information and change "Unknown" to "Mate" to be correct.
Do you think this is a bug in the Lucas Chess or it's something else that I don't know? I'm a beginner with chess programs and just many years ago played with Chessmaster 9000. Since that time, I forgot everything about all complex chess software. I use the last version of Lucas Chess (Version R 1.27a).

Best
Masoud