Sadly AtomicMap is one of the features that isn't as paid attention to as it probably should. It doesn't work with the binary storage or storeAsBinary from previous versions. You can go ahead and log a JIRA if you wanted.
Okay. I'll log an issue on JIRA. But it may be less priority. I just read docs somewhere telling that AtomicMap can be used as a distributed MultiMap.
For now, I just replace the logic as a List/Set/Map in the value.