Hi friends.
I know why this bug is happening. If you mod the aircraft armour shield value, giving a number below +4, the game take it as a percentage. For example, i changed the value to +2 but the game shows 100%, not +2. When i put again on +4, the game show that value and the aircraft repair itself at base.
So, if you mod the aircraft shield armour, don't give values below +4.