Hi @simlev ,
in case you're still wondering about this, here's the FB documentation about their Advanced Matching functionality, if you check the 'Description' column in the 'Customer Information Data Parameters' section, you can see that nearly every field is supposed to be sent lowercased (sometimes with spaces and puncutation removed as well).
That's because JOHN, John, jOhN, and john (etc.) will all produce different hashes because of the differences in capitalization - it makes much more sense for FB just to store the hash of 'john' instead of every possible variant's hash.
Does that help?