Single-value datetime fields duplicating on index

[katherine-boost]

Schema block:

datetime :syndication_date do
    store false
    solr_name 'syndication_date_ds'
    search_as %i[filter]
    search_value do |record|
      syndication_date = record.dates.select(&:syndication_date?)
      syndication_date = syndication_date.first.parse_as_solr_date_range if syndication_date.present?
      syndication_date.present? ? DateTime.parse(syndication_date) : record.created_at
    end
  end

The value delivered by the search_value block is a single DateTime, e.g. Fri, 05 Jun 2020 15:41:42 NZST +12:00.

When calling .index! on that record, this error message:

Error: {
  "responseHeader": {
    "status":400,
    "QTime":4},
  "error": {
    "metadata": [
      "error-class","org.apache.solr.common.SolrException",
      "root-error-class","org.apache.solr.common.SolrException"],
    "msg": "ERROR: [doc=SupplejackApi::Record 5ed9bef6400382484286ff9b] 
            multiple values encountered for non multiValued field syndication_date_ds: 
           [2020-06-05T03:41:42Z, 2020-06-05 15:41:42 +1200]",
    "code":400
    }
}

Both the UTC and NZST varieties of the DateTime are being delivered.

Removing search_as %[filter] fixes the problem.

It looks like a bug was introduced in #223.
Prior to that (and when rolling back to there), it was working fine.