Question Mirror Shader only showing white surface on iOS 15

Not sure whether this belongs here or in the iOS subforum, but we are having issues with a mirror shader recently on iOS. It just shows a white surface instead of the mirror on iOS 15 (and also strangely on a device running iOS 13). It works fine on iOS 14 and any other platform like Android / Mac Standalone and also in the editor.

I’m working on a MacBook with M1 chip since a couple of months and also changed Unity version from 2020.3.9f1 to 2020.3.22f1 at about the same time. So for me there are 3 possible sources for the issue: my new M1 dev machine, iOS 15, Unity update.

My question: Is anyone experiencing the same or has a solution? I googled for other mirror shaders but couldn’t find any.. the shader we are using is called “Ciconia Studio/MirrorReflection”. I also wrote to the developers but didn’t get an answer. Any help is greatly appreciated.