The "Admin - Remove a member from a blob store group" task does not work if the source blobstore is s3

1. Configure a repository with an s3 blob store, and upload some content to it.  
2. Promote the blob store to a group.
3. Create a new file based blob store
4. Add this new blob store into the group
5. Configure and run an "Admin - Remove a member from a blob store group" task to remove the s3 blob store from the group.

The blob store will be removed from the group, but the content will not be moved out of it. A simple test with a blobstore that contains one file shows:

blobstore.group.memberRemoval-20190531131122168.log:2019-05-31 13:11:23,153-0500 INFO [quartz-3-thread-9] SYSTEM com.sonatype.nexus.blobstore.group.internal.tasks.BlobStoreGroupMemberRemovalTask - Elapsed time: 985.5 ms, *Total processed 0/4

There are no errors in the log.

INFO and DEBUG task logs attached.