SERVFAIL if nameservers are not reachable in IPv6 but they are in IPv4

Ok, so it seems 1.1.1.1 does not failover to IPv4 if a nameserver do not respond over IPv6

dig mechanicus.it @\1.1.1.1

; <<>> DiG 9.10.3 <<>> mechanicus.it @\1.1.1.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 31590
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1452
;; QUESTION SECTION:
;mechanicus.it.                 IN      A

;; Query time: 4591 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Sat Sep 01 14:34:36 ora legale Europa occidentale 2018
;; MSG SIZE  rcvd: 42

dig mechanicus.it @\1.0.0.1

; <<>> DiG 9.10.3 <<>> mechanicus.it @\1.0.0.1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 19113
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1452
;; QUESTION SECTION:
;mechanicus.it.                 IN      A

;; Query time: 4916 msec
;; SERVER: 1.0.0.1#53(1.0.0.1)
;; WHEN: Sat Sep 01 14:34:45 ora legale Europa occidentale 2018
;; MSG SIZE  rcvd: 42

dig mechanicus.it @\8.8.8.8

; <<>> DiG 9.10.3 <<>> mechanicus.it @\8.8.8.8
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62958
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;mechanicus.it.                 IN      A

;; ANSWER SECTION:
mechanicus.it.          14399   IN      A       185.56.218.11

;; Query time: 250 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sat Sep 01 14:34:50 ora legale Europa occidentale 2018
;; MSG SIZE  rcvd: 58

dig +short CHAOS TXT id.server @\1.1.1.1 “MXP”
dig +short CHAOS TXT id.server @\1.0.0.1 “MXP”

http://dnsviz.net/d/mechanicus.it/dnssec/

My ISP AFAIK do not provides name resolution over IPv6.
Nameservers:

Everything was fine until few days ago.

Not sure if the same, but I observed a similar thing recently - IPv6 timeouts appear to be racey - #2 by lukastribus

It could be the same, after IPv6 timeout there isn’t an IPv4 attempt apparently.

As an update, 1.0.0.1 seems to work sometimes, with multiple queries about the same domain.
When 1.0.0.1 get the result in cache, then also 1.1.1.1 serves it.