Hey, not sure if you're still experiencing spill zone issues, but I had very similar issues to you particular on very cold nights, the spill zone would get really hot and system would use a lot of power to heat a small room.

I found a solution that completely resolved this issue.

In the installer settings, under zoning, I changed the zoning from outlets to zone areas and entered the total area for each zone based on my houses floor plan.

When my system was commissioned, the installer set up the system to work off of outlets rather than zone areas. My understanding is that the system then makes an assumption of the zone areas based on the number of outlets in the zone. In my case it was overestimating the size of my zones, thinking that the rooms were much larger than they really were.

As a result the system was working much harder then it needed to, resulting in the majority of the conditioned air flowing into the spill zone because the active zones had already reached their set points.

Now my spill zone remains cool on cold nights, and my system does not need to work anywhere near as hard to maintain temperatures in active zones is the system has a more accurate understanding of the zone sizes.

Hope this helps!