I have CiviCRM 5.13 on Drupal7 with the clickatell v4.1 extension for SMS. Clickatell integration works, I can send test SMS via HTTPS from the browser to test phones. Database and environment is set to UTF-8.
However, if a mass SMS is compiled in https://example.com/civicrm/sms/send?reset=1, or a direct SMS is sent, the non-English characters, like é,á,... are lost or transformed in the SMS body. If the same text is sent over HTTPS/curl directly to Clickatell, there is no such issue.
Is it a CiviCRM issue, or a misconfiguration on my side? How can I track it down?