Time sync works but is not persistent

[turbhrus]

I'm setting my inverter (Deye SUN-6K-SG04LP1-AU) time with an automation loosely based on the example in the readme.
It works, but after 30 seconds or so the time reverts to whatever it's currently set to in the inverter, which has quite a bit of drift.

The "time syncs" box on the inverter front panel is unchecked. Something else is overriding the time.
Any ideas?

Here's the automation for reference.

alias: Inverter time sync
description: Set device's time to now() formatted for the inverter
triggers:
  - at: sensor.sun_next_dawn
    trigger: time
  - event: start
    trigger: homeassistant
actions:
  - data:
      datetime: "{{ now().strftime('%Y-%m-%d %H:%M:%S') }}"
    target:
      entity_id: datetime.deye_inverter_date_time
    action: datetime.set_value
mode: single 

[nazar-pc]

Is is connected to the Internet? Might be best to block its access on the router so it can't speak to the cloud and receive commands to do who knows what.

It also doesn't support NTP or other standard protocol, so the only way to "knows" about the time is from the cloud it connects to. I suspect this is where it gets the date/time from. Maybe firmware bug that renders the switch not functional on the inverter or something.