A teammate adds a SPLADE index to an existing BM25 and...

RRF treats all input lists as independent votes. When you add SPLADE to a system already containing BM25, you are effectively giving the lexical signal two votes against the dense signal’s one. This biases the ranking toward lexical matches, which hurts performance on paraphrase queries where dense retrieval is superior. By grouping the lexical signals first, you restore the balance between the lexical and dense retrieval families.