Here is dump of mongo shell.

1. some documents have "jclient.targets_list" == []
   > db.mondata.find({"jclient.targets_list": {$exists: true}}).count()
   578244
2. some documents have no "jclient" at all
   > db.mondata.find({"jclient.targets_list": {$exists: false}}).count()
   3943
3. it's indexed field
   > db.mondata.getIndexKeys()
   [
   [skipped] { "jclient.targets_list" : 1 }

   ]

4. and.... here is "distinct"
   > db.mondata.distinct("jclient.targets_list")
   [ null, null ]

It really looks like bug.

Here are some more strange samples:
> db.mondata.distinct("jclient.targets_list", {"jclient.targets_list": {$exists: true}})
[ ]
> db.mondata.distinct("jclient.targets_list", {"jclient.targets_list": {$exists: false}})
[ null ]

1. so [] + [null] should be [null], should not it?
   > db.mondata.distinct("jclient.targets_list", {$or: [{"jclient.targets_list": {$exists: false}}, {"jclient.targets_list": {$exists: true}}]})
   [ ]