Render template seems to have problems with special characters

[afarago]

What happened?

When render_template is called it fails with special characters and changes them to a strange 0xff characters.

Adding any of this to the render engine should yield to 21.8 °C

{{states('sensor.nspanel_diningroom_temperature', rounded=True, with_unit=True)}}
{{states('sensor.nspanel_diningroom_temperature', rounded=True)}} {{'%c'|format(176)}}C
{{states('sensor.nspanel_diningroom_temperature', rounded=True)}} °C

Instead it renders as 21.8���C

Relevant code lines -- see addTimeText:

    def update_time(self, kwargs):
        time = datetime.datetime.now().strftime(self._config.get("timeFormat"))
        addTemplate = self._config.get("timeAdditionalTemplate")
        addTimeText = apis.ha_api.render_template(addTemplate)
        self._send_mqtt_msg(f"time~{time}~{addTimeText}")

Application is nspanel ui, latest release (4.3.3)
https://github.com/joBr99/nspanel-lovelace-ui

Version

4.4.2

Installation type

Home Assistant add-on

Relevant log output

-----------------------------------------------------------
 Add-on: AppDaemon
 Python Apps and Dashboard using AppDaemon 4.x for Home Assistant
-----------------------------------------------------------
 Add-on version: 0.15.2
 You are running the latest version of this add-on.
 System: Home Assistant OS 11.1  (aarch64 / raspberrypi3-64)
 Home Assistant Core: 2023.11.2
 Home Assistant Supervisor: 2023.11.3

Relevant code in the app or config file that caused the issue

def update_time(self, kwargs):
        time = datetime.datetime.now().strftime(self._config.get("timeFormat"))
        addTemplate = self._config.get("timeAdditionalTemplate")
        addTimeText = apis.ha_api.render_template(addTemplate)
        self._send_mqtt_msg(f"time~{time}~{addTimeText}")

Anything else?

No response