Say you have two statuses, OPEN and START PROGRESS.
OPEN has the jira.permission.assignable.projectrole property set to role_A.
START PROGRESS has the jira.permission.assignable.projectrole property set to role_B.
Transitioning from open to start progress, there is a transition screen with the Assignee field. If you try to assign to someone who is in role_B, the assign will fail with error "cannot be assigned issues". (despite the auto-complete manages to show the user as an assignable user!)
This is not the intended behavior because in JIRA 6.3.4 or older, you can assign to a user in role_B without problems, since users in role_B matches the workflow property configured in the START PROGRESS status, which is the destination of the transition.