https://dom.spec.whatwg.org/#concept-node-replace

[[
Insert node into parent before reference child with the suppress observers flag set.

Let nodes be node's children if node is a DocumentFragment node, and a list containing solely node otherwise.

Queue a mutation record of "childList" for target parent with addedNodes nodes, removedNodes a list solely containing child, nextSibling reference child, and previousSibling child's previous sibling. 
]]

In the case that the new node is a DocumentFragment, the Insert step removes node's children (per https://dom.spec.whatwg.org/#concept-node-insert, Step 5). Then "let nodes be" is always the empty list, and then the addedNodes in the mutation record is empty.

Perhaps "let nodes be" should come before "insert node into parent."