updateZoneKeyRange is inconsistent with updating and deleting tags when tag ranges are prefix for shard key

mongos> sh.shardCollection( "test.c" , {a: 1, b: 1}) { "collectionsharded" : "test.c" , "collectionUUID" : UUID( "5c33de36-7f4a-4cc0-bdf1-b8101a7d34b1" ), "ok" : 1, "operationTime" : Timestamp(1573048485, 13), "$clusterTime" : { "clusterTime" : Timestamp(1573048485, 13), "signature" : { "hash" : BinData(0, "AAAAAAAAAAAAAAAAAAAAAAAAAAA=" ), "keyId" : NumberLong(0) } } } mongos> use admin mongos> sh.addShardToZone( "shard01" , "zone1" ) { "ok" : 1, "operationTime" : Timestamp(1573048597, 1), "$clusterTime" : { "clusterTime" : Timestamp(1573048597, 1), "signature" : { "hash" : BinData(0, "AAAAAAAAAAAAAAAAAAAAAAAAAAA=" ), "keyId" : NumberLong(0) } } } mongos> db.runCommand({updateZoneKeyRange: "test.c" , min : {a: 1}, max: {a: 2}, zone: "zone1" }) { "ok" : 1, "operationTime" : Timestamp(1573048606, 8), "$clusterTime" : { "clusterTime" : Timestamp(1573048606, 8), "signature" : { "hash" : BinData(0, "AAAAAAAAAAAAAAAAAAAAAAAAAAA=" ), "keyId" : NumberLong(0) } } }   mongos> db.runCommand({updateZoneKeyRange: "test.c" , min : {a: 1}, max: {a: 2}, zone: null }) // Command succeeds but doesn't delete the tag range. { "ok" : 1, "operationTime" : Timestamp(1573049199, 1), "$clusterTime" : { "clusterTime" : Timestamp(1573049199, 1), "signature" : { "hash" : BinData(0, "AAAAAAAAAAAAAAAAAAAAAAAAAAA=" ), "keyId" : NumberLong(0) } } } mongos> db.getSiblingDB( "config" ).tags.find() { "_id" : ObjectId( "5dc2d11e8bc365c6f4466713" ), "ns" : "test.c" , "min" : { "a" : 1, "b" : { "$minKey" : 1 } }, "max" : { "a" : 2, "b" : { "$minKey" : 1 } }, "tag" : "zone1" } mongos> db.runCommand({updateZoneKeyRange: "test.c" , min : {a: 1, b: MinKey}, max: {a: 2, b: MinKey}, zone: null }) // Deletes the tag range. { "ok" : 1, "operationTime" : Timestamp(1573049663, 1), "$clusterTime" : { "clusterTime" : Timestamp(1573049663, 1), "signature" : { "hash" : BinData(0, "AAAAAAAAAAAAAAAAAAAAAAAAAAA=" ), "keyId" : NumberLong(0) } } } mongos> db.getSiblingDB( "config" ).tags.find({min: {a: 1, b : MinKey}}) mongos>