Question about Music Server Function



I am using Kodi mainly to provide me with a music server. I have lots of files ripped from my CDs and the structure is like this

folder flac – artist name – album names

folder mp3 – artist name – album names

so yes, in principle I have all albums double once in flac (for use in my home network) once as mp3 (when i’m using mobile network). 

now when i browse the folders on a mobile device most of the albums are displayed separately, for some albums though mp3 files and flac files are merged together in one album, which is kinda annoying…does anybody have an idea, how to fix this? 

Would be really great, as I am pretty clueless about it…