This can happen if the username is longer than 8 characters (OR) id has no name. But, If you want the username in the ps output then try this,
ps
ps -eo uname:20,pid,pcpu,pmem,sz,tty,stat,time,cmd | grep '[b]ash'
You can parse out the /proc entry if you are on Linux and if you just need a numeric pid (or are OK with it). Here is an example for the mysqld process:
/proc
mysqld
grep -e '^Uid:' /proc/$(pidof mysqld)/status | cut -f 2
The shortest way I found so far ( $PID - ID of the process inspected):
ps -p $PID -o euid=