I actually did have quite a chat with an Apple support team member and she was kind enough to point out the fact that, although i DID have a service that pushed my content to icloud, it doesn't seem to have been icloud music library, but a hybrid between the simple iCloud back up and iCloud music library. In some ways, for some reasons, when i subscribed to apple music, that initial service was updated to icloud music library and linked to my apple music subscription.
It sounds very strange, but that's what I managed to understand from all the things she said. Although she did point me to some other ways of trying to recover my music (initially, for some reason, thinking it's an iPhone issue, not a mac issue), then we even tried tricking icloud into giving back my stuff through trying to recover artist/album art and updating icloud music library from File-Library-Update iCloud Music Library. None of these worked.
Finally, she registered a formal complaint with Apple tech team, requesting the software engineers to review the possibility of this being a bug. I'm not sure if she actually did do that or if she told me this in order to reassure me...
What DID work in the end was me subscribing to Apple Music AGAIN, when immediately all of my media became available for download. And I immediately downloaded everything and made a copy of my iTunes media folder, so that when I cancel my subscription again tomorrow everything will not be lost again.
Once more, tt2, thanks for trying to help me, but unfortunately the only thing that did work for me was resubscribing, downloading items, cancelling subscription. iCloud music library is just a great, yet poorly implemented feature that, in my opinion, should definitely work when you have a big iCloud space subscription, not just when you have an Apple Music subscription. It would just complement every other collaborative feature between mac&iPhone&iPad&apple TV, so yeah, I do think it should "just work"!