I've needed an array_filter over multiple values but seen in \Drupal\migrate\MigrateExecutable::processRow that this is only possible with a plugin that defines handle_multiples in its *definition* (not configuration), so Callback will not do.
Code a CallbackMultiple process plugin.
Doit.
Postponed: needs info
6.0
Plugins
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
The current single value plugin doesn't solve the problem when we need to pass in an array directly to the callback. I was trying to run "array_unique" and pass an array with "single_value", but the "callback" plugin would process the individual values, not the whole array. It raises the question whether core's plugin should actually accept multiple values.