This bug is related to a change in the response behavior of BIND 9.9.5. See https://deepthought.isc.org/article/AA-01113/0/Case-Insensitive-Response-Compression-May-Cause-Problems-With-Mixed-Case-Data-and-Non-Conforming-Clients.html
The problem is not in BIND 9.9.5 but that check_dig is case-sensitive in what it accepts! -- Reply to this email on GitHub: https://github.com/monitoring-plugins/monitoring-plugins/issues/1233#issuecomment-34491074
