It definitely seems to be a bug - I've had the same issue as Athildur, where - without moving my character, without using any stances, and without any other apparent changes in battle conditions - a melee attack that previously correctly required 3AP suddenly required 4AP. The only way I could then "fix" the issue was to take one tiny step closer to the enemy - but we shouldn't have to do that.
Yes, you should. If you stand too far from an enemy, approaching him costs you the first AP. Due to heavy movement penalities even small distances are AP-heavy for melees...
