See RDA Appendix G (Titles of Nobility, Terms of Rank, Etc.) and also Appendix A for the language involved. In this case the title is French so it is not capitalized. When the title is British, it is capitalized. That’s why the capitalization varies in the name authority file.
Policy and Standards Division
Library of Congress
I recently had cause to use the following authority record. I plan on updating the record and removing the 667 note but I am not sure about the capitalization rules regarding the royalty term "baron". I noticed when searching the naf that sometimes the term is capitalized and sometimes it's not. I couldn't find an RDA rule or explanation regarding when it should be capitalized and when it shouldn't be. Any help would be appreciated. Thanks.
010 n 50023126
040 DLC ǂb eng ǂc DLC ǂd DLC ǂd Uk ǂd DLC
046 ǂf 1768 ǂg 1830
1001 Fourier, Jean Baptiste Joseph, ǂc baron, ǂd 1768-1830
4001 Fourier, Joseph, ǂc baron, ǂd 1768-1830
667 THIS 1XX FIELD CANNOT BE USED UNDER RDA UNTIL THIS RECORD HAS BEEN REVIEWED AND/OR UPDATED
670 His Die Auflösung der ... 1902
670 His Joseph Fourier face aux objections contre sa théorie de la chaleur, 1980: ǂb t.p. (Joseph Fourier)
670 Lavoisier, A.L. Elements of chemistry, 1985: ǂb t.p. (Jean-Baptiste-Joseph Fourier)