i contacted the support for this issue and had a little chat. But they weren't quite able to help me, other than telling me things that i dont know and can understand. So i hope that there is someone who can give some technical information about that.
Here's the bug/problem as you wanna call it: if the device is oriented in vertical screenorientation, the data saver option is always on. You can disable it and scroll down to change the quality of the streamed song/podcast and you are not able to change it. Then you scroll up the option is on again, even without closing the settings and opening them again. The bug is not there in horizontal screenorientation and thats where i'm wondering why.
I mean, how the heck should the screen orientation influence the settings file of spotify on the device? It would mean that it constantly overrides it so it stays on.