If the domain does not exist (NXDOMAIN) an SPF client MUST return
"unknown".
This is the correct behavior. Not all domains are public or in public
DNS's, duplicate lookups don't matter coz of local result caching, and
it's not the job of SPF to decide for the admin what to do with fake
domains - such decisions already exist elsewhere in everything, so
adding it to SPF is bad.
Anyway - we all know that SPF should avoid feature-creep and settle
down, stop changing, and "get out there". Every time you update
anything, you turn away load of people who decide "SPF must be just
another set of unprofessional, dangerous, untested ideas by wannabe
antispammers".
Stick to the core.
Chris