When I empty my watch towers for repurposing the man-power on other tasks, then save the game (usually I leave 4 guards on their posts from 40 open slots), exit the game and finally reload the game in order to continue, all guard posts will be fully manned (40/40). My expectation is that the man-power on the guard posts remains the same after re-entering the game.
Another bug (or feature) that I discovered with watch towers: when I want to assign two guards on a tower while still having some towers completely unmanned, the game will automatically fill the first slot of each watch tower until it will fill the 2nd slot that I wanted the game to fill. Usually I get attacked from only one side of my town by raiders. So I want to maximize the damage of my towers on one side of the town. If the game tries to fill up all towers it takes a lot of time to find free workers that all start looking for weapons. In the end I get my second guard to the tower when the raid is already over. My expectation is that I can fill a 2nd slot without needing to fill all other open 1st slots first.