What exactly is the assignment you are talking about?
If the assignment is something like: "Upgrade some Frosty Fjords zones" (I don't know if this is the exact English assignment, I'm not playing the English version), then it does not mean you have to upgrade any building in the Frosty Fjords region, it means you have to upgrade any Frosty Fjords zone or building regardless in which region this zone exists.
So, upgrading normal residential zones or Tokyo zones or Paris zones or... in the Frosty Fjords region will not help to fulfill the assignment.
Upgrading a Frosty Fjords zone placed e.g. in the Green Valley region will help.
I understand the point you are making and you are correct the assignment is to upgrade a specific named zone. Whilst my capital city does have mixed zones of the older type, ie; Paris, London, Omega etc this is not the case in the newer regions. I’ve only placed Frosty Fjord in that region, Sunny Isle in that region etc. It’s not a case of accidentally upgrading the wrong zone as, for now, I’ve only placed the native zone. It does seem to be that sometimes it recognises the upgrade and sometimes it doesn’t.
I’m assuming upgrading a zone involves placing additional new foundations and then upgrading the building. There are also assignments that say upgrade a building of a specific type and for these you usually can just upgrade any qualifying building that isn’t a foundation.
