Not a bug, the Night Witches are just a special case! (again.)
Other hangars (B-17, C119, and Nato Hangars) allow you to train planes on demand, and in order to do so, guarantee an extra space in your plane limit. Since the Night Witches are only available once every 8 hours, the rules change slightly to make sure you don't miss the chance to add them to your army and throw off the cadence of their availability.
For example:
Your Night Witch Hangar has a plane ready to claim. If you have fewer than your max number of planes, this Night Witch counts towards your total number of planes.
However - if you have trained as many planes as you can, and then claim the available Night Witch tactic, it will briefly allow you to increase your hangar space by +1 to fit that new Night Witch. This allows the countdown clock to begin again so the next Night Witch can be available in 8 hours.
TL;DR
Train all your planes, then claim the extra Night Witch for maximum air superiority.
/Closed