R8 or ProGuard obfuscates method names and may remove methods, which are not called (dead code removal). Because Subscriber methods are not directly called, ProGuard assumes them to be unused. So if you enable ProGuard minification, you must tell ProGuard to keep those Subscriber methods.
Use the rules specified in the project README in your R8/ProGuard configuration file (proguard-rules.pro) to prevent Subscribers from being removed.
Note: You will need this configuration regardless if you are using a subscriber index or not.