If there is a valid specified "Map Directory To" we should use it instead of overriding it with the source folder mount point.
extract the project that is zip in the UDN forum link provided
drop a fbx into the folder /Game/Assets/Props/
The content monitor should propose to import the file
Import the file
RESULT
The file is import in /Game/Props
EXPECTED RESULT
The file is import in /Game/Assets/Props
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-51524 in the post.