Mass update selecting first available value

I am working on Cases module at my company , we are using suitecrm 7.
In the Cases module we have added extra fields
each case now has a Type, Category and Sub category
But i am facing this issue, whenever i try to mass update the cases, reassining the cases to some other person, then the sub category value changes and it takes the first available value, i searched on other topics that this might be a bug in suitecrm, can somebody help me with this issue, i don’t want to change the value of sub category when doing mass update.