@gsmet I just found this: https://docs.jboss.org/hibernate/stable/search/reference/en-US/html_single/#d0e7456 See the information box about null values, in particular.
It seems it can be considered as a feature or a bug, depending on how you interpret this piece. But overall, it mainly states that the distance to null coordinates will always be excluded from search results when executing a spatial query, and that unexpected (though deterministic) stuff will happen when executing a non-spatial query.