I'm using the PHP wrapper to send custom field data with Subscriber.AddWithCustomFields.
In the backend there is a custom field with multiple options. Two of these options contain special characters (ø and é, specifically). Even though the values of the array I pass in through Subscriber.AddWithCustomFields are verified UTF-8, the options are simply not enabled for that subscriber. Other options that do not contain these characters work just fine.
I have the same problem. Hebrew characters become gibberish, while any other date looks fine.
I know the original problem here was a few months ago, but in case you're still looking for a fix: