I have a site to site VPN setup between my vacation home and my home base via unifi. all works well for accessing each home assistant on each network. I have an automation that let me know when I enter a zone and leave a zone.
When I arrive at my vacation home it says I have entered but then shortly after it says I left (even though I clearly did not). it does this multiple times when i stay in the zone for the weekend. When I am at my home base it does not do this. I have reviewed all of my automations and looks to be correct.
Assuming the zone sizes are the same, does your holiday home have worse cell phone coverage?
When indoors your phone can not reach the GPS satellites and uses cell tower triangulation for location. This is a lot less accurate than GPS and depending on the number of cell towers nearby can vary in accuracy.
If there are fewer cell towers near your holiday home your phone location could be wandering outside the zone due to this.
There are many ways around this but the best way is to use more than one tracker for your person entity. e.g. phone tracker and NMAP or Ping or Private BLE. Lots of presence trackers you can try listed here: Integrations - Home Assistant and device trackers are here: Integrations - Home Assistant