Per me si:

My configuration:
Windows 2012 R2
Endian 3.0.5 Beta1
Proxy with NTLM non transparent.

After the setup to connect Endian to the DC I had to modify the squid.conf.tmpl adding this lines after the NTLM section:

#UTM-929 LT
auth_param negotiate keep_alive off
auth_param ntlm keep_alive off

I also have added a WINS server to the DC and added a Proxy DNS Routing from Endian to the DC.

PC WIN7 (domain) IE, FF, CH -> no authentication windows -> OK
PC WIN7 (no domain) IE, FF, CH -> authentication required (use the format “domain\user”) -> OK