[Koha] 648$a not correct indexing

Dear list, The data which we saved in 648$a (2019, for exm.) in Koha versions 17.11, 17.05, 18.11 couldn't show the results when we search as "subject" . Why? Can anybody advice how to save chronological term in 648 field and show the correct "subject" search results? Thanks, Erlanda