I include a birth date of 1848 on the Merchant Navy search and get a 2 page response from 1700s to 1800s BUT nothing close to 1840s.
FamilySearch treats dates loosely, and when there is no exact match fills in Search results with approximate matches.
I tried this out using the example of a "John Smith" (date of birth 1848) who might be found in the Merchant Seaman Records collection for the UK. Without checking the "Exact" fields I received 2,768 results (different years), in seemingly random order. Unfortunately, checking on a birth date alone produces no results - one has to include a place name, too. However, I found nothing for 1848 (for a John Smith) in using any search method, so tried another way. I removed the first name and unchecked the box alongside (you must do this if a field is empty). This produced just the one result for a (Robert) Smith born in (Norfolk) England in 1848. This shows that (using the required criteria - in this case: last name, year and place of birth, all checked as "exact") results can be narrowed down. In your example, maybe there is just nobody by that name / year of birth included in the database concerned.
See https://www.familysearch.org/search/record/results?count=20&q.birthLikeDate.from=1848&q.birthLikeDate.to=1848&q.birthLikePlace=england&q.birthLikePlace.exact=on&q.surname=smith&q.surname.exact=on&f.collectionId=1762440 for how I produced the result detailed above.