If you remove the offset, does it trigger at sunset or 1 hour after sunset?
If it triggers 1 hour after sunset, there may be a problem with the configuration of your system clock (the timezone and/or DST for your location is incorrect).
So when you remove the offset it triggers exactly at sunset?
If it does trigger at sunset without the offset option, then it means both timezone and DST offset are correct and the problem may be due to the format of the offset value.
Try this instead (note the leading zero):
trigger:
- platform: sun
event: sunset
offset: '-01:00:00'
If it still fails to trigger one hour before sunset, check the automation’s trace.