External tilesets are supported, but relative paths (
../) are not because they do not point to valid paths within an app bundle. There's no "going up by one folder" in an app bundle. Or maybe there is but only if you setup the folder references within Xcode accordingly.
It looks like you have the TMX in folder A and the tilesets in B/tilesets/terrain_01/Terrain.tsx
You should have the TMX at the root folder, and tilesets in the same or subfolders to prevent the use of relative paths. If you do use subfolders relative to the TMX file, make sure you add these folders as "folder references" in Xcode (should have a blue icon).
Let me know if you can't work this out, I may not be explaining this intuitively.
Udpate with example, this is difficult to achieve (and currently not possible):
I'll look into whether "parallel" folders can be made to work using xcode folder references and provide an example if it does work.