JIMIN by BTS just released a new song track promise. When is this coming to spotify? His track was streamed in soundcl*ud 15M times in just 3 days. Surely, spotify can easily add this to their song list.
I have been wondering this as well. I don't have the exact answer in my hand at the moment (I am not a Spotify employee either) but I'm guessing it has to do with copyright issues and earning permission from the artist himself and their company.
I am pretty sure Spotify discusses whether or not they can upload a song on their own system. But this process is probably time consuming and cumbersome since there are copyright issues and such. This is the case with a few other artists. I don't have a lot of examples in my head but one is "Fortune Cookie" by Jonghyun of SHINee. This song was a bonus and exempt track from his debut solo album BASE and it hasn't been put on Spotify to this date (the album was released in 2015).
RM's RM mixtape isn't on Spotify either, so I think they are both ongoing processes with both Spotify and BigHit itself.