SPA8000 and DNS SRV Resource Record

I have a linksys SPA8000 that should register on a SIP router.
He should reach the SIP  registrar via SRV record (for redundancy purpose).
On "Proxy Server" i set the domain where he should connects to.
 Actually, by default, SRV is enabled, but registration fails.
Dumping the traffic i look that Linksys send DNS SRV query on the domain correctly, but he don't ask for _sip._udp.  So, the DNS return nothing and registration fails.
I had to set DNS SRV Auto Prefix, then the registration worked.
SPA8000 sends correctly a DNS SRV query with _sip._udp.domain.tld.

Cisco SPA describes this entry:

DNS SRV Auto Prefix
If enabled, the SPA9000 will automatically prepend the Proxy
or Outbound Proxy name with _sip._udp when performing a
DNS SRV lookup on that name.
Default: no

Dunno why someone wants to use SRV record without query to _sip._udp.
Probably is for old, legacy devices.