This is normal behavior. The AI won't necessarily prioritize enemies by range--there are a number of factors that filter into targeting preference, and if they are currently blocked from reaching their target they'll wait until something changes, be it the surrounding circumstances (an ally clearing the way for them, a new path opens up, etc) or a target priority shift.
Although it doesn't happen too often, this can also mean two enemies blocking each other in a hallway won't necessarily immediately fight, but once one of them eventually opens fire on the other, it'll inevitably cause a target priority change and they'll both open up (but in Cogmind most bots are weak enough that the first to fire probably wins anyway).