I'm so glad you looked into this and managed to break it down.
I have been experiencing the same behavior (First with subsonic) and now Airsonic for years. There were similar posts about this issue in the past but it was never resolved.
I resorted to adding the hidden folders on my pooled disks containing my music in order for Airsonic to detected them on a scan, however this meant I had 2-3 copies of everything in my library.
I am looking forward to a fix!