Okay, that confirms what I thought it was supposed to do for standard cost. (Except that PartPlant isn’t merely an optional override; it must exist.)
This is not what it does with the materials in a configured MoM. It appears to grab the first/any PartCost with a matching PartNum, ignoring CostID. I could not replicate the issue when quoting a part with a static/non-configurable MoM. I’ll try to replicate with a minimal configurable MoM.