this was a user-contributed part.
Not sure, if changing
$decoded = json_decode($value, true)
to
$decoded = $this->modx->fromJSON($value);
would make a difference
I replace half of the corresponding condition in ...core/components/migx/model/migx/migx.class.php but still see some unreadable abracadabra instead of Cyrillic words while trying to edit them