Shards, which are created by a higher versions, cannot be allocated to lower version nodes (in some cases). We had the same issue with shards from version 6 which did not get allocated to ES 5 nodes 
Shards, which are created by a higher versions, cannot be allocated to lower version nodes (in some cases). We had the same issue with shards from version 6 which did not get allocated to ES 5 nodes
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