If I navigate to /cgi-bin/koha/admin/searchengine/elasticsearch/mappings.pl and try reordering fields under the 'Biblios' or 'Authorities' tabs, after clicking 'Save' the fields are still where they are to begin with.
That's weird... Just tested on master branch 17.12.00.021 : reordering is correctly saved on 'Biblios' tab but not on 'Authorities' tab.
I'm able to reproduce this also with latest master.
*** This bug has been marked as a duplicate of bug 18827 ***