Sadly it's not a bug just terrible system, Shisui BT is not hard to get so your protections spent are already higher than average. Don't worry tho this game is so amazing that there are people like me who spent way over 5000 protections for 10T Madara breaks and still he is not maxed. You would think that pity rates are something that exists in most games but not this one, if you are unlucky it's over.