Looking more into this function -- scanning the documents and viewing old posts -- I've decided to update this answer. Since the behavior itself existed way before FCM was officially released, a lot of things have already been updated (some deprecated) in the service (under the hood). An example of this is going way back to C2DM to GCM migration, the part where the tokens weren't interoperable was stated.
For using getToken() by passing comma-separated senderIds, seeing as it is not documented or is advised/mentioned in any official FCM documentation or statements, it is best to avoid using them and instead proceed with the official procedure -- by using getToken("senderId", "scope") for each sender and maintaining each of those tokens as needed.
Choosing to still use getToken() by passing comma-separated senderIds may probably produce unexpected behaviors in the future, so I strongly encourage developers to proceed with the suggested procedure in the duplicate post.
