If somebody had the Delux Edition (which includes Artbook and Soundtrack), why the standalone soundtrack isn't checked with tick for being owned? Standalone soundtrack says 42 mp3 tracks and Deluxe Edition Soundtrack has 42 tracks in either MP3 or FLAC. What's the deal?

I'm pretty sure those are just some oversights on GOGs end. Buying the soundtrack should give you both the MP3 and FLAC versions.