Murillo opened a WRC case about this and we are working to purge the messages on his system. The PurgeActivityData task is not the task which purges Ensemble messages, as Vic noted.

Another aspect of this issue is that pre 2017.1 SOAP Wizard-generated messages do not have %OnDelete generated and any persistent properties are orphaned on delete. We will need to implement %OnDelete to make sure that persistent properties are cleaned up as well.