process plugin geofield_latlon misparses data from d6_node_complete + get [#3163198]

Problem/Motivation

migrating d6 site to d8
old d6 content-type contains separate latitude and longitude fields
new d8 content-type should contain a geofield
geofield_latlon process plugin is supposed to enable this conversion
however geofield_latlon misparses its input when fed from d6_node_complete + get

Steps to reproduce

migrate_plus.migration.upgrade_d6_node_complete_club.yml contains:

# ...
class: Drupal\node\Plugin\migrate\D6NodeTranslation
field_plugin_method: null
cck_plugin_method: null
# ...
source:
  plugin: d6_node_complete
  node_type: club
# ...
process:
  # ...
  field_latitude:
    -
      plugin: get
      source: field_latitude
  field_longitude:
    -
      plugin: get
      source: field_longitude
  field_geofield:
      plugin: geofield_latlon
      source:
        - field_latitude
        - field_longitude
# ...

Add var_dump($value) before first line of transform() in web/modules/contrib/geofield/src/Plugin/migrate/process/GeofieldLatLon.php

Run drush migration:import

transform() receives values like:

array(2) {
  [0]=>
  array(1) {
    [0]=>
    array(2) {
      ["value"]=>
      string(9) "50.854334"
      ["delta"]=>
      string(1) "0"
    }
  }
  [1]=>
  array(1) {
    [0]=>
    array(2) {
      ["value"]=>
      string(9) "-0.553837"
      ["delta"]=>
      string(1) "0"
    }
  }
}

First line of transform() is: $value = array_map('floatval', $value);

This misparses the data as supplied.