Precondition:

1. Have Network discovery rule with SNMP check and have it checked in Host name parameter:
   

Steps to reproduce:

1. Open that pre-saved Network Discovery rule form
2. Change SNMPv1 check's port
3. Note that Host name setting hasn't changed and SNMPv1 still left there
4. Save form
5. Reopen form

Result:
Host name setting changed to DNS name.

This is reproducible in 6.0 and 6.4 Zabbix versions, but not in master.