The domain souissi.net is in the ISC DLV registry.
From Unbound 1.2.0, I can get the MX or the SOA:
% dig +dnssec MX souissi.net
...
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 3, AUTHORITY: 3, ADDITIONAL: 9
...
But not the RRSIG:
% dig +dnssec RRSIG souissi.net
...
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 45731
...
BIND 9.5.1 has no problem getting the RRSIG