I’m not sure, but I got it fixed – I emailed Marc Gravell who developed protobuf, and he said that before the Serialize(…) and Deserialize() methods, there shouldn’t be [ProtoBeforeSerialization] and [ProtoAfterDeserialization], respectively. Though I’m not sure what broke between it working on Android and it not working on Android using the same code. Maybe it was something in an Android or Xamarin update.
Cheers for the help though!