You could try using the
-addconfigurationdebugging rule that was added in ProGuard 6.0.
I’m not sure it will help, but it should print some runtime suggestions that you can use and it has helped me in the past when I had weird issues. (make sure you disable it when you release your app)
Copied from the docs:
Specifies to instrument the processed code with debugging statements that print out suggestions for missing ProGuard configuration. This can be very useful to get practical hints at run-time , if your processed code crashes because it still lacks some configuration for reflection. For example, the code may be serializing classes with the GSON library and you may need some configuration for it. You can generally just copy/paste the suggestions from the console into your configuration file. Counter-indication: do not use this option in release versions, as it adds obfuscation information to the processed code.