I have an assembly which had all models/parts fixed. Using Fix constraint.
Why did you not Constrain the Components of your Assembly? If you had, the parts would not have moved.
Any modifications at part level should not affect the Fix constraint at assembly level. There may be any other reason that you are facing this issue. Lot of things depends how do you use it. It would be great if you capture (from NX) and share a short video clip of this issue. It will be easy to understand the issue and to help you.
Guessing, did the Bend unbend move the part relative to absolute CSYS of part, if so that would explain it, fix constrains the absolute csys to it's current position in the assembly, if the solid moves in its own part csys it will move in the assembly.