The method _select_relevance_score_fn is incorrect, as pointed out by a community member who opened up a PR to address one case.

https://github.com/langchain-ai/langchain-mongodb/pull/99

As I commented there, we did not see this earlier because we don't use the method. The first thing to try is simply to remove it as check that all of our tests pass. We'll perform due diligence, but it does not look like it is used elsewhere in langchain either.