Random quality loss in offline playlists

[user-removed]

I am connected to wifi, and listening to a playlist I have saved offline. The audio quality is good, while suddenly it drastically drops to a very low quality. I can feel the change very well, and when I go back to the player and scroll back, it changes back to good. I set the streaming quality to automatic, but the playlist is offline, so slow wifi shouldn't be a problem.

However, when I'm not connected to internet and am not in offline mode (I never use it) quality never drops.

How do I fix this? I have a suspicion spotify streams even offline playlists when connected to internet. Is there a way to make it play local songs from the device instead of from server?