Problem with is_answer and replies without question section

[Philip-NLnetLabs]

An older version of unbound has the following bug:
$ dig +tcp +bufsize=0 +nocookie +keepalive www.nlnetlabs.nl

; <<>> DiG 9.18.12-0ubuntu0.22.04.1-Ubuntu <<>> +tcp +bufsize +nocookie +keepalive www.nlnetlabs.nl
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 60049
;; flags: qr tc rd ad; QUERY: 0, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; Query time: 0 msec
;; SERVER: 185.49.141.38#53(185.49.141.38) (TCP)
;; WHEN: Fri Apr 14 15:46:35 CEST 2023
;; MSG SIZE rcvd: 12

This causes is_answer to return false. For TCP transports I would like to return this error. I could write a custom is_answer locally. But maybe not checking the question when there is no question section should be supported.