If one of the directories in the dump is a symbolic link to another directory (outside the dump) containing bson files, the directory is ignored, and nothing within it is restored. The legacy version follows the symlink and restores the data (using the name of the symbolic link, not the directory it links to, as the db name).

I do see a log message

don't know what to do with file "dump_with_soft_link/dbTwo", skipping...

I tried the same with a collection bson file as a soft link, and the file was restored correctly with the rewrite.