I had noticed this before, but thought maybe it was a fluke or caused by other variables like player attributes or other aspects of the play, but I havent seen the flat assignment work once.

In this example the defense is running from dime formation with the CBs and NBs in Man, the Mike in zone, and both safeties in flat.
The offense is running from pro set, the FB is motioning Right wide streak, the TE is running slant, the RWR is motioning left slot right out, the LWR is running streak, and RB is running Flat Left.
As you can see in the picture both safeties stay completely still, even as the RB comes out of the Flat. no one picks him up. Ive seen the exact same results from Flat assigned CBs, LBS, and D-lineman, and against other routes.
Im not sure what the Flat coverage assignment is intended to do, but as its currently operating its seems to me like you'd have to be crazy to use it because the assigned defenders just freeze. Is this a bug or am I missing something?