In 2.2.0-rc0 tag aware sharding behaves strange: it doesn't offer strict guarantee to which chunk a document will be placed when inserted. So if we have 2 tags for 2 shards (e.g. 'US-West' and 'US-East') on an empty collection, inserting 2 documents which map to different tags will place both documents into one chunk; and the chunk location will depend on to which of the tags we've made insertion first. This behavior scales to any number of documents: there's always a high chance to have documents "misplaced" this way after automatic chunk splitting.