when you export JSON from the Trello website, you have to save the Trello JSON in anyone of Unicode format (UTF-8) to keep the Unicode chars.

Please ensure that you have saved the JSON file in Unicode format and the problematic chars are existed in the problematic JSON file and check the case again.

If you still face the issue, please send us the problematic Trello JSON file and send us a log file (from the Settings tab -> Export Logs) for further analysis.