Rage - you're a tester. Why don't you test these defenses on the test server (you have my ok to do so - just make sure you document what you are doing on the test server). If you can show me that in every instance the defender does not go to a secondary assignment, then we'll consider it a bug. Until then, it's going to be as Bort stated - working as intended as players are failing vision checks.
This right here shows that it is possible for it to be one way one play and another a different play:
http://goallineblitz.com/game/replay.pl?game_id=1514353&pbp_id=2270878http://goallineblitz.com/game/replay.pl?game_id=1514353&pbp_id=2267085For now - locking this pending test results.