Order of operations is important.
Changing a connector, seam, etc all force the item to redevelop which is why you're loosing the oversize options.
The redevelopment is important to properly calculate the shape to cut. If you change a connector for a S&D to TDC or Flange Out, it changes the amount of material that's needed. That's why the redevelopment is required.
Item.Update() recalculates that info.
So, any changes you edit to a fitting should be done before anything that would alter the default development pattern like oversize options, editing notches, adding pierce points, etc.