It appears this AS is on the tail of
7018 10910 12124 15188
701 10910 12124 15188
AT&T (7018)
InterNAP (10910)
Thorn.net (12124)
UUNET (701)
Who isn't filtering?
InterNAP. They are large enough that their transits usually don't filter
them, yet they have had several problems in the past with their
customers and not validating the information they've been given. It's
even possible that InterNAP is filtering but took Thorn's word for it.
I'm unfamiliar with 12124's history.