If you got here from the good first issue label, see this comment.

For example, the following elasticsearch.yml ...

node.name:
- node-a
- node-b

... starts up a node called [node-a, node-b]:

[2018-08-24T17:08:43,807][INFO ][o.e.n.Node               ] [[node-a, node-b]] initializing ...

In fact this is what's happening in #30946. The following API call...

PUT _cluster/settings
{
  "transient" : {
    "cluster.routing.allocation.exclude._name" : [ "elasticsearch-data-0" ]
  }
}

... sets cluster.routing.allocation.exclude._name to the string [elasticsearch-data-0], which then does not match the node called elasticsearch-data-0 so the allocation is not filtered as expected.