Why does ps aux displays a number instead of a username

Issue

• Unable to view username on ps output

ps -ef  | grep -i testusers 

• It is working with user id of that user

ps -ef | grep -i 512

• Why username is not displaying in ps -ef command.

[root@localhost ~]# ps -ef | grep testusers
root     23278 22322  0 04:54 ?        00:00:00 sshd: testusers [priv]
512      23280 23278  0 04:54 ?        00:00:00 sshd: testusers@pts/1
root     23303 22890  0 04:55 pts/2    00:00:00 grep testusers
[root@localhost ~]# 
[root@localhost ~]# getent passwd testusers
testusers:x:512:513::/home/testusers:/bin/bash
[root@localhost ~]#