I think it's a feature rather than a bug. Military units have a zone of control and cannot be forced to move. Agents can always be pushed out of their map tile by a military unit. Therefore, when they are pushed so that there is nowhere to move, it is logical that they simply die.
So it's just a feature resulting from the collision mechanics of campaign map characters. Similarly, the fact that you can use ships to block the crossings around Constantinople is also a feature of the campaign map collision mechanics.