On a sidenote, the safe spots for different type weapons stated on the official website are incorrect. The correct safe spots are as follows:
White Items: +3 (can whipe or destroy on a +4 attempt)
Type A Items (purple): +2 (can whipe or destroy on a +3 attempt)
Type B Items (intense): +1 (can whipe or destroy on a +2 attempt)
Type C Items (orange): +0 (can whipe or destroy on a +1 attempt)
So it is quite possible that your intense +1 weapon whiped on a +2 upgrade attempt.
White Items: +3 (can whipe or destroy on a +4 attempt)
Type A Items (purple): +2 (can whipe or destroy on a +3 attempt)
Type B Items (intense): +1 (can whipe or destroy on a +2 attempt)
Type C Items (orange): +0 (can whipe or destroy on a +1 attempt)
So it is quite possible that your intense +1 weapon whiped on a +2 upgrade attempt.