It seems Location (relative) column behaves like "plain" Location column in user-collections showing full paths even though only last few segments of paths differ. But in Find collection (and possibly Duplicate Files, didn't test) Location (relative) column works as expected. If I add both columns in my own collection they both show same thing (non-relative paths). Please investigate.
This is normal.
If you do a Find Files starting from some folder, Opus makes a note of it in the collection, so that the Location (Relative) column knows what all the paths are relative to.
If you just create a collection and drag some files to it, the collection does not have that, and the files could come from anywhere, so they aren't relative to any folder (even if the files you manually drag into the collection happen to all be below a particular folder).
Would it make sense to expand Location (relative) column to take into account paths of all displayed items everywhere it's used? Now this column is accessible everywhere but only useful in search results collection.