I think I found a bug.
Twice now a garrison of mine got unlocked by being attacked, and got then placed OUTSIDE of the town they were guarding. At start of the turn they were inside of the structure.
Once the enemy stayed in the region (and took over the town), once the enemy was beat off, and the garrison stayed in possesion of the region (and the now empty town).
I think this is not WAD....
It does not always happen, as I also have had some garrisons defend the town as expected. I don't remember if those became unlocked or were already unlocked before though.
(see also "more strange results" in the main RoP forum)