Offline mode turns itself off

[bogod]

Plan

Premium

Country

 UK

Device

OnePlus 6t

Operating System

(Android Pie / OxygenOS)

My Question or Issue

Offline mode only seems to stick while I have the app open. If I go to a different app or turn my screen off while my (offline) music is playing, when I come back to Spotify I'll find that offline mode has turned itself off. Why is this? And how can it be remedied? Offline mode is an important feature for anyone who does not want to run up unexpected bills witht their network, but it's useless in the form I've just described..