This is getting stranger by the minute. Using ncat/telnet from one of my servers (instead of my workstation) I can connect using uppercase PASS and NICK commands.
This issue seems to be limited to my workstation. Small dump for future reference:
Ubuntu 16.04, kernel is 4.4.0-47-generic
The libc used by telnet/ncat: GNU C Library (Ubuntu GLIBC 2.23-0ubuntu4) stable release version 2.23