Again I'm just a stupid engineer..... But why "are they clearly torque to yield bolts" Does the discription of the Part# say that??? Do you have the part spec???
Torque to yield bolts are used where there is lots of themal expansion and you need even presure on ...oh say, a gasket seat(or don't you read) And it would sure be the first time I've seen a torque to yield bolt, that's cadium plated. ???????
If I was to guess.... I would say you have Alum. sandwiched between steel. After you hit the spec torque and once you go through some body roll, that torque is but a memory because the Alum. has extruded(you can see this if you look around the holes)At that point vibration could make the bolt wiggle loose without a lock.
This would be a perfect application for a split lock washer. Instead they buy thread lock by the 55 gallon drum, because the engineers don't want the added weight and the bean counters don't want the added expense and the service folks can charge you a couple saw bucks(x8) for a $1 part..... so everyone is happy.... even the folks who think Bentleys is the word of God, instead of a reprint of BMW T.I.S