If you are using a Rule to do this, then the rule will ask for the device’s location when you select either Sunrise or Sunset. That location stays fixed wherever your phone is.
If you use the Plug’s Schedules menu item (Plug 2021 and above), then your phone should be set to the Plug’s time zone when you create it. After that I believe it will use that time zone’s sunrise and sunset as long as you don’t edit the schedule when elsewhere.
I believe this is also dependent on your phone’s time zone when you enable the feature. So whenever setting up any sunset/sunrise schedule, always be sure your phone is set to the same time zone.
So the “Timer” setting has not been turning off the light after whatever you have it set to? It should be submitted in a Fix-it (first) Friday bug if so.
In the meantime, there is a trick you may be able to use. If you have a spare Plug or Bulb lying around you can use them as a proxy to turn off the light. What you do is set up a Rule that says if the camera with the spotlight detects motion (which turns the spotlight on), then turn on the Plug or Bulb. Then you set a second Rule that says if the Plug or Bulb has been on for x minutes, turn the Spotlight plus the Plug or Bulb off. Bonus is if you light a light, then you can see when the spotlight is on from inside the house. 