enumConfig.xml

<enumsConfig>
  <enum name="auState">
    <value>ACT</value>
    <value>NSW</value>
    <value>NT</value>
    <value>QLD</value>
    <value>SA</value>
    <value>TAS</value>
    <value>VIC</value>
    <value>WA</value>
  </enum>
</enumsConfig>

schema.xml

<?xml version="1.0" encoding="UTF-8" ?>

<schema name="facet-bug" version="0.1">
  <field name="id" type="long" indexed="true" required="true" stored="true" multiValued="false" />
  <field name="name" type="cistring" indexed="true" stored="true" multiValued="false" />
  <field name="state" type="auState" indexed="true" multiValued="false" docValues="true" />
  <field name="insertdate" type="date" indexed="true" multiValued="false" />

  <fieldType name="date" class="solr.TrieDateField" />
  <fieldType name="long" class="solr.TrieLongField" precisionStep="0" positionIncrementGap="0"/>
  <fieldType name="cistring" class="solr.TextField" sortMissingLast="true">
      <analyzer type="index">
        <tokenizer class="solr.StandardTokenizerFactory"/>
        <filter class="solr.LowerCaseFilterFactory"/>
        <filter class="solr.ReversedWildcardFilterFactory" />
      </analyzer>
      <analyzer type="query">
        <tokenizer class="solr.StandardTokenizerFactory"/>
        <filter class="solr.LowerCaseFilterFactory"/>
        <filter class="solr.ReversedWildcardFilterFactory" />
      </analyzer>
  </fieldType>
  <fieldType name="auState" class="solr.EnumField" enumsConfig="enumConfig.xml" enumName="auState" />

  <uniqueKey>id</uniqueKey>
  <defaultSearchField>name</defaultSearchField>
</schema>

solrconfig.xml

<?xml version="1.0" encoding="UTF-8" ?>
<config>
  <luceneMatchVersion>5.2.0</luceneMatchVersion>

  <requestHandler name="standard" class="solr.SearchHandler" default="true">
    <lst name="defaults">
      <str name="echoParams">explicit</str>
      <int name="rows">100</int>
      <str name="fl">*</str>
      <str name="version">2.1</str>
    </lst>
  </requestHandler>
</config>

testdata.json

[
      {
        "id":2,
        "name":"asd",
        "state":"QLD"},
      {
        "id":3,
        "name":"asd",
        "state":"WA"},
      {
        "id":1,
        "state":"QLD",
        "name":"HelloWorld",
        "insertdate":"2016-01-06T23:37:36.655Z"},
      {
        "id":12,
        "state":"QLD",
        "name":"HelloWorld2",
        "insertdate":"2016-01-06T23:37:48.023Z"}]

testrun.json

"First testrun (using facet.field) works, second one (using JSON api) does not"

curl http://localhost:8983/solr/core1/select -d 'q=*:*&wt=json&indent=true&facet=on&rows=0&facet.field=state&facet.field=insertdate'
{
  "responseHeader":{
    "status":0,
    "QTime":20,
    "params":{
      "q":"*:*",
      "facet.field":["state",
        "insertdate"],
      "indent":"true",
      "rows":"0",
      "wt":"json",
      "facet":"on"}},
  "response":{"numFound":4,"start":0,"docs":[]
  },
  "facet_counts":{
    "facet_queries":{},
    "facet_fields":{
      "state":[
        "QLD",3,
        "WA",1],
      "insertdate":[
        "2016-01-06T23:37:36.655Z",1,
        "2016-01-06T23:37:48.023Z",1]},
    "facet_dates":{},
    "facet_ranges":{},
    "facet_intervals":{},
    "facet_heatmaps":{}}}

curl http://localhost:8983/solr/core1/select -d 'q=*:*&wt=json&indent=true&rows=10&
json.facet={
  states: {field: state}, dates: {field: insertdate}
}'
{
  "responseHeader":{
    "status":400,
    "QTime":12,
    "params":{
      "q":"*:*",
      "json.facet":"{\n  states: {field: state}, dates: {field: insertdate}\n}",
      "indent":"true",
      "rows":"10",
      "wt":"json"}},
  "response":{"numFound":4,"start":0,"docs":[
      {
        "id":2,
        "name":"asd",
        "state":"QLD"},
      {
        "id":3,
        "name":"asd",
        "state":"WA"},
      {
        "id":1,
        "state":"QLD",
        "name":"HelloWorld",
        "insertdate":"2016-01-06T23:37:36.655Z"},
      {
        "id":12,
        "state":"QLD",
        "name":"HelloWorld2",
        "insertdate":"2016-01-06T23:37:48.023Z"}]
  },
  "error":{
    "msg":"Expected numeric field type :state{type=auState,properties=indexed,stored,docValues}",
    "code":400}}

It works when the enum field is dropped from the JSON query:

~/d/s/l/solr ❯❯❯ curl http://localhost:8983/solr/core1/select -d 'q=*:*&wt=json&indent=true&rows=10&
json.facet={
  dates: {field: insertdate}
}'
{
  "responseHeader":{
    "status":0,
    "QTime":1,
    "params":{
      "q":"*:*",
      "json.facet":"{\n  dates: {field: insertdate}\n}",
      "indent":"true",
      "rows":"10",
      "wt":"json"}},
  "response":{"numFound":4,"start":0,"docs":[
      {
        "id":2,
        "name":"asd",
        "state":"QLD"},
      {
        "id":3,
        "name":"asd",
        "state":"WA"},
      {
        "id":1,
        "state":"QLD",
        "name":"HelloWorld",
        "insertdate":"2016-01-06T23:37:36.655Z"},
      {
        "id":12,
        "state":"QLD",
        "name":"HelloWorld2",
        "insertdate":"2016-01-06T23:37:48.023Z"}]
  },
  "facets":{
    "count":4,
    "dates":{
      "buckets":[{
          "val":"2016-01-06T23:37:36.655Z",
          "count":1},
        {
          "val":"2016-01-06T23:37:48.023Z",
          "count":1}]}}}