Bug reports.

By "specified plane," I meant a static (unchanging) plane. And, frankly, this is a flaw. If it affects the rest of the gameplay so much, create a separate version of the rules or something where the teleport specifically targets the same plane for mirrors.

This isn't a bug. In the game, each object has not 2, but 3 coordinates. The nearest town is calculated based on the sum of the coordinates, and the one on the surface has a sum that is 1 less, so it's all logical, in my opinion. In Warlords, this was often used as a feature.