Fill multi-value field from comma separated values in a field

I have data in xml which I want to import into drupal. Which works fine. But one field in the xml is filled with a comma separated list of terms. There is a target field in drupal which is a multi-valued field. I do not need those as a taxonomy, just as multi-value for solr search. I just want to expand the one,fourty,flower,... values to fill the multiple-value field.

It absolutely will not work. Any help?

Here is the latest iteration (I tried a lot in the last two days) which fails spectacularly with an error like so:

Error: Unsupported operand types in /var/www/dev.diercke.com/web/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php on line 206 #0 /var/www/dev.diercke.com/web/core/modules/migrate/src/MigrateExecutable.php(399): Drupal\migrate\Plugin\migrate\process\SubProcess->transform(Array, Object(Drupal\migrate_tools\MigrateExecutable), Object(Drupal\migrate\Row), 'field_stichwort...')
#1 /var/www/dev.diercke.com/web/core/modules/migrate/src/MigrateExecutable.php(205): Drupal\migrate\MigrateExecutable->processRow(Object(Drupal\migrate\Row))
#2 /var/www/dev.diercke.com/vendor/drush/drush/includes/drush.inc(206): Drupal\migrate\MigrateExecutable->import()
#3 /var/www/dev.diercke.com/vendor/drush/drush/includes/drush.inc(197): drush_call_user_func_array(Array, Array)
#4 /var/www/dev.diercke.com/web/modules/contrib/migrate_tools/src/Commands/MigrateToolsCommands.php(958): drush_op(Array)
#5 [internal function]: Drupal\migrate_tools\Commands\MigrateToolsCommands->executeMigration(Object(Drupal\migrate\Plugin\Migration), 'karte', Array)
#6 /var/www/dev.diercke.com/web/modules/contrib/migrate_tools/src/Commands/MigrateToolsCommands.php(418): array_walk(Array, Array, Array)
#7 [internal function]: Drupal\migrate_tools\Commands\MigrateToolsCommands->import('karte', Array)
#8 /var/www/dev.diercke.com/vendor/consolidation/annotated-command/src/CommandProcessor.php(257): call_user_func_array(Array, Array)
#9 /var/www/dev.diercke.com/vendor/consolidation/annotated-command/src/CommandProcessor.php(212): Consolidation\AnnotatedCommand\CommandProcessor->runCommandCallback(Array, Object(Consolidation\AnnotatedCommand\CommandData))
#10 /var/www/dev.diercke.com/vendor/consolidation/annotated-command/src/CommandProcessor.php(176): Consolidation\AnnotatedCommand\CommandProcessor->validateRunAndAlter(Array, Array, Object(Consolidation\AnnotatedCommand\CommandData))
#11 /var/www/dev.diercke.com/vendor/consolidation/annotated-command/src/AnnotatedCommand.php(311): Consolidation\AnnotatedCommand\CommandProcessor->process(Object(Symfony\Component\Console\Output\ConsoleOutput), Array, Array, Object(Consolidation\AnnotatedCommand\CommandData))
#12 /var/www/dev.diercke.com/vendor/symfony/console/Command/Command.php(255): Consolidation\AnnotatedCommand\AnnotatedCommand->execute(Object(Drush\Symfony\DrushArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#13 /var/www/dev.diercke.com/vendor/symfony/console/Application.php(1027): Symfony\Component\Console\Command\Command->run(Object(Drush\Symfony\DrushArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#14 /var/www/dev.diercke.com/vendor/symfony/console/Application.php(273): Symfony\Component\Console\Application->doRunCommand(Object(Consolidation\AnnotatedCommand\AnnotatedCommand), Object(Drush\Symfony\DrushArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#15 /var/www/dev.diercke.com/vendor/symfony/console/Application.php(149): Symfony\Component\Console\Application->doRun(Object(Drush\Symfony\DrushArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#16 /var/www/dev.diercke.com/vendor/drush/drush/src/Runtime/Runtime.php(118): Symfony\Component\Console\Application->run(Object(Drush\Symfony\DrushArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#17 /var/www/dev.diercke.com/vendor/drush/drush/src/Runtime/Runtime.php(48): Drush\Runtime\Runtime->doRun(Array, Object(Symfony\Component\Console\Output\ConsoleOutput))
#18 /var/www/dev.diercke.com/vendor/drush/drush/drush.php(72): Drush\Runtime\Runtime->run(Array)
#19 /var/www/dev.diercke.com/vendor/drush/drush/drush(4): require('/var/www/dev.di...')
#20 {main}
Error: Unsupported operand types in Drupal\migrate\Plugin\migrate\process\SubProcess->transform() (line 206 of /var/www/dev.diercke.com/web/core/modules/migrate/src/Plugin/migrate/process/SubProcess.php).
 [warning] Drush command terminated abnormally.

...
process:
...
  field_stichworte: tags
  field_stichworte_einzeln:
    - plugin: explode
      source: tags
      limit: 10000
      delimiter: ,
    - plugin: deepen
    - plugin: sub_process
      process:
        field_stichworte_einzeln:
          - plugin: extract
            index:
              - 0
...