Relocating Planes Mid-Flight: Bug or Intentional Behaviour?

The aircraft behavior you described is related to the "attack" order, you gave to the stack. Instead, order them to be "on patrol". They will destroy their target likewise (by circling over the air strip) but afterwards they are allowed to land on their new destination without a tank stop at the former air strip.

Rule:

attack order --> after attack the aircraft have to refuel on the air strip they departed from

patrol order --> after patrol the aitcraft do not have to refuel; if the aircraft are landing on a new air strip, they have to refuel

To answer your question: in my opinion the aircraft behavior is intentionally. It avoids an advantage for the attack order.

How is it an advantage for the attack order? The patrol mechanic is already too overpowered as it is.

Aircraft handling in CoW knows one very vulnerable and dangerous situation: refueling the aircraft. In this situation the aircraft is more or less defendless. The rule says, refueling must follow up an aircraft attack or when relocating the aircraft to a new landing spot. So, if you want to attack and relocate your aircraft, you have to refuel two times according to the rule. Your suggestion was to refuel only once by relocating the aircraft right after an attack. In your case you would have "saved" one refueling process.