So if I try to re-scan just the album folder. I would get all 1-15 tracks except for the two I mentioned before. And then the message "Did not important (2 files) message".
Also, as mentioned, I have chosen to use this one album as an example but in fact, for the last rescan I have done, it has chosen not to import several tracks from different albums. Again, same file type, and genres etc and using the methodology I explained above, to check whether they are "Recently Added", I see that some are missing.