This is a bug in the implementation, plain and simple. I have no idea why it was implemented this way; the whole business of using LazyList and adding to it seems like classic premature optimisation but the bottom line is that this is clearly just a property setter for a collection of transformer functors. But there's no way to remove an element from the collection, so it should not be additive.
As such, this will get fixed en passant by the and changes, but I will record it separately anyway.
I have no idea whether this is a bug or note but the method
Is additive, not replacing.
Contrast the use of the same helper method in
ItemMetadataAddingStage