> if we have several hits with the same text match score, I would expect them to be sorted by vector similarity That's not how rank fusion works. Rank fusion uses the rank of the document in keyword search and combines that with rank of the document in semantic search and

alpha

parameter for weighting both components. So even if several hits have the same text match score, if there is no secondary sorting condition, they have to be ordered somehow -- which in our case is done by the ID of the record (document indexing order).