When would you search with a hypothetical answer rather than the question, and what is the risk?

An answer that works

When questions and passages are written differently enough that a question embedding sits far from the passage that answers it. You ask a model for the answer it would expect and search with that, so you are comparing a passage to a passage. The risk is that the guess is specific and wrong: an invented detail becomes part of the query and pulls the search towards documents containing that detail rather than the correct one. Measure recall and MRR separately, because the usual damage is to the ranking rather than to whether the right chunk is retrieved at all.