I don't precisely know how and why did this happen but it probably has something to do with joining Training Flights. I joined a Training Flight room with my Hawk III and wasn't even put into a team since the room owner was afk/occupied whatever. After realizing he wasn't present I left the room and joined other room where I was put into a team. The battle couldn't be started because my Hawk was obviously in battle (yellow timer symbol next to plane model).
And the plane is still "In Battle". None of the training rooms I were in even exist anymore.