Ping from VLAN only with FQDN

Hello Community,

i have a problem.

on my xg i have 2 Networks:

1. 172.20.10.x (Default LAN)
2. 172.20.8.x (VLAN8)

Both have his own DNS Server:

172.20.10.x  --> DHCP with DNS 172.20.10.1
172.20.8.x --> DHCP with DNS 172.20.8.1

i added all my Hosts in DNS of Sophos.

if i Ping from 10.x Network all works fine. From 8.x Network i cannot Ping the normal name. I can ping only if i set a dot (.) behind the normal name. 
BTW: NSLOOKUP works fine.

so for Example from VLAN 8:

ping computer01  --> doesn't work

ping computer01.  --> works

nslookup computer01 --> works

i have two Network Rules between VLAN8 and DEFAULT LAN with ANY ANY.

So nothing is blocked here.

Can anyone help please?