It seems to me that group_concat_max_len is not being read from /etc/my.cnf.
I have this in /etc/my.cnf, in the [mysqld] section: "group_concat_max_len = 65536". But when the server re-starts, it goes back to the default of 1k (too small!). Copying and pasting that same string into mysql (prepended with SET GLOBAL) does work.
Upon stopping and re-starting the server, nothing shows up in the log to indicate that /etc/my.cnf may have not been read all the way through.
I looked around the website, but could see no documentation for which system variables are honoured in /etc/my.cnf, and I didn't see any way to set this via mysqld arguments, so I have to wait until someone tells me a web page look weird before going in through mysql to fix it!