City Rank Loss but not Tech bug.
When a town (rank 5) drops to village (rank 4) the tech only available to towns is properly greyed out and unselect able and Tech points are reduced appropriately.
However the functionality of tech previously chosen is still enacted.
We can still craft T4 materials, enchant, and use the marketplace.
Kind of makes sense...;once you've discovered a technique, you can keep using it, even if you lose your city rank, however, if you lose rank, you can't develop new tech because of your limitations.
The intention was to loose the ability or benefits from the tech, this is not what they intended. A city degraded to rank 1 should not have all the tech it unlocked when it was rank 15.
Unless the building built from that tech tree investment is somehow destroyed, I see no reason why people shouldn't be able to not keep using it for whatever it can do.
Imo this is a bug.