Steps to reproduce:{}

1. Prepare chroot environment

2. Add a user with a uid that does not exist in the host system to this environment:
# cat /chroot/etc/passwd | grep vmal
vmal:*:9001:0:vmal:/home/vmal:/usr/local/bin/bash

# cat /etc/passwd | grep -c 9001
0

3. Run the process in the chroot environment as this user:
# chroot /chroot/ su -l vmal -c top

# ps aux | grep top
root   22047   0,0  0,0   45320  2548  1  I    13:26             0:00,02 su -l vmal -c top
9001   22048   0,0  0,0   16596  2520  1  S+   13:26             0:00,68 top

Result:

Since the user with this uid does not exist in /etc/passwd on the host system, proc.get returns the user's uid instead of the user name:
# zabbix_agentd -t 'proc.get[top,,top,thread]'
proc.get[top,,top,thread] [s|[\{"pid":2699,"ppid":2698,"jid":0,"name":"top","jname":null,"user":"9001","group":"wheel","uid":9001,"gid":0,"tid":294405,"tname":"top","cputime_user":0,"cputime_system":0,"state":"sleeping","ctx_switches":1098,"io_read_op":0,"io_write_op":0}]]

# zabbix_agentd -t 'proc.num[top,,sleep,top]'
proc.num[top,,sleep,top] [u|1]

However, this value cannot be used further to obtain information about the process:
# zabbix_agentd -t 'proc.get[top,9001,top,thread]'
proc.get[top,9001,top,thread] [s|[]]

# zabbix_agentd -t 'proc.num[top,9001,sleep,top]'
proc.num[top,9001,sleep,top] [u|0]

Expected:
Using user ID as a parameter for user name may be allowed if no user with that name is found.