This may be intended or unintended, I don't know.
AFAIK the behavior used to be that if a colonist had a melee weapon, they would strictly use that melee weapon to attack. If a colonist had no melee weapon, they would alternate attacks between arms.
I'm seeing my colonists use their bionic/arcotech arm to attack even when equipped with a plasteel long sword, thereby reducing the dps wasting every other attack with the bionic arm. If there's no bionic arm, the colonist uses the long sword for every attack.
EDIT: This only seems to occur when outside the main colony map, such as in random encounters and quest locations.
I think it's mostly intended...
Bionic or not, they will still use their fists even with a plasteel longsword. Since i think bionic arms for the pawns they have higher damage output than the fists.
But i still get pawns using their fists even with a superior melee weapon, even without bionic parts.
It's intended. They evaluate all their attacks and tend to use the better ones but there is always some random mixture between their different attack verbs.