I have couple of reports from the users of my module that the preferences set in the settings part of a module don't stick. Unfortunately, I don't have any technical details about devices where that happens nor logs, but couple of users were the owners of Nexus 6P. I guess it has something to do with SeLinux or some other security restrictions. Could it be related to e.g. systemless rooting method? I run my module on couple of marshmallow nexus devices without any issues. My devices have modified kernel to allow classic SuperSU. I'm not using systemless root.