there seems to be some copy&paste error in the romwelten list in group2(you might want to poke YomanROM Keili Slyco or meisjustme for it, whomever you can grab first) since one leg item is listed with "stats 5" which would be a non-red stat. "Stats 5" belongs to the shield.
Loot group1 with the weapons seems correct, in loot group 2 all legs have a red stat guaranteed, the shield does not.
So the chance is (assuming equal chance and that my math does not fail me right now) 1:7 to get a non-red stat at all at the last boss.
This was also already part of a
bug report on DE.
For now: Shield in Loot = no red stat.