EDIT: I’ll mark this down as solved for now since the issue seemingly solved itself. 
I’ll revisit if it shows up again but I’m leaving the topic here too in case someone runs into the same thing in the meantime.
Just started using N15 and ran into a bug - works as expected in N14 and earlier so it’s a new issue. When I import audio into a project, the files end up on separate tracks even though I’ve chosen “Import Files on One Track”. If I use the Import > Audio File command, it works as expected, so it’s specific to Mediabay (which is what I use in my workflow). I’ve tried resetting the preferences, same problem.
Can anyone replicate this?