Xiaomi Mi A2 Lite
My Question or Issue
I have encoutered this issue and many post with people with problems rebooting the phone and mantaining the storage option properly configured as SD card.
I've found the solution! (at least to my case).
I hope it can help someone else.
I've bought this new phone some weeks ago, and it has Android Pie (my old phone was Marshmellow) and in this version I've set a PIN number and, for some reason, didn't let configured to ask for the PIN before the system boots (with is the most secure option). Instead, it asked for the PIN before the first unlock and after the system boot. That's the catch:
I noticed that the SD card only mounts AFTER I unlock the first time after the boot. You can see it by the SD card icon who apears quickly at status bar just after the unlock. For security reasons, it make sense, since someone who stole the phone could try to load malicious software via external storage. Spotify service probably loads BEFORE the SD card mount, and then, like a mother who call the police when his son is 30 seconds late to come back home, it switches to the internal memory and starts to download all again. If the PIN is typed before the system boot, the SD card mount and when Spotify service loads, it can find the external storage. Works like a charm.
This solution did not let Spotify devs off the hook, since many other apps that use external storage didn't present this issue at all.
Make sure, if you have a PIN or password set, to Android ask BEFORE the system boot.