I call this a bug... Is it?

[Tuidjy]

Here is the setup. Two huge enemy armies, one sitting in a castle,
the other in the open. A task force teleports and besieges the
castle. On the same time, the armies move against each other.
The army in the castle someone avoids the siege, somehow misses
the incoming horde, and the two armies end up trading places.

I call it a bug. And a rather serious one, that I think just
lost me a game. I realize that one could say that this is just
the engine working as designed, i.e. armies moving into friendly
provinces. What do you think?