What makes you think that "foo.a.x" is delegated when "a.x" is?
1. My feeling for what should be ...
2. The RFC.
3. Testing with other nameservers, running bind and powerdns... if i rebuild my configuration there, both answer with NS records for foo.a.x, what i actually expected...
As i wasn't able to reproduce the behaviour of the he.net nameservers, i wonder what's wrong on which side...
[later] ... so i tested a bit more and:
4. The he.net nameservers are giving me different results for the same query... ns1 answers with the delegation, ns2..5 are using the wildcard... just got even stranger...
edit: Now ns1 is also using the wildcard... testing with
dig +norecurse @ns1.he.net asdf.whyst.trds.de