It does not surprise me, Microsoft also prescribes very accurately how (any) scenery project directories are structured, handling of terrain files (.cgl) could be on a per package basis.
Q: have you looked into the console… what’s the error
Q: why would you put more than one terrain file in a package ?
Maybe your .CGL files could be merged ? By the way, FSDeveloper/breadeater developed a parser for CGL files, there could be some documentation for your question,