This happens when tag is 10 instead of 010. One simple way to reproduce is to change the URL. staff_URL/cgi-bin/koha/admin/marc_subfields_structure.pl?tagfield=10&frameworkcode= yields the same as staff_URL/cgi-bin/koha/admin/marc_subfields_structure.pl?tagfield=100&frameworkcode= instead of staff_URL/cgi-bin/koha/admin/marc_subfields_structure.pl?tagfield=010&frameworkcode=
This feels like a WONTFIX to me as this should only happen if the URL is manipulated manually. Please reopen if you disagree.