Tectonic appears to apply a broken version of its world gen modifications to other mod dimensions, such as the Lost Cities dimension. While the terrain generates in smooth Tectonic fashion, The Lost Cities I think depends on flatter terrain to make its Cities look like... well, cities. Currently the dimension includes large carved out chunks to make the terrain flat. It also is missing grass and flowers on generated grass blocks on the terrain. This may affect other modded dimensions, like Twilight Forest or Aether, but I only had time to test and confirm in Lost Cities.

Is it possible to disable Tectonic's modifications to other dimensions? I would prefer to restrict it to the Overworld only.

Edit: This is on Tectonic 2.4.1, running on Minecraft 1.20.1 with Forge 47.3.0.

The Lost Cities simply references the Overworld files. There is not a way for me to fix this issue without either:
A: Making the code significantly more complex by moving any references to overworld files into separate tectonic-namespaced files (this comes with a brand new set of issues that would be a headache to resolve and would likely keep reappearing)
B: Specifically adding a compat patch for Lost Cities.

Won't fix.