NextDNS over IPv6 is very slow and times out continuously

Posted this as comments in existing bug reports, but got a response that this issue warrants a separate report.

dig/nslookup using either the primary or secondary give timeouts:

$ nslookup google.nl 2a07:a8c1::xx:xxxx
;; communications error to 2a07:a8c1::xx:xxxx#53: timed out
Server: 2a07:a8c1::xx:xxxx
Address: 2a07:a8c1::xx:xxxx#53
Non-authoritative answer:
Name: google.nl
Address: 142.251.36.35
;; communications error to 2a07:a8c1::xx:xxxx#53: timed out
;; communications error to 2a07:a8c1::xx:xxxx#53: timed out
Name: google.nl
Address: 2a00:1450:400e:810::2003
$ dig +tcp @2a07:a8c1::xx:xxxx google.nl
;; Connection to 2a07:a8c1::xx:xxxx#53(2a07:a8c1::xx:xxxx) for google.nl failed: timed out.
;; no servers could be reached
; <<>> DiG 9.20.7 <<>> +tcp @2a07:a8c1::xx:xxxx google.nl
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 63615
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;google.nl. IN A
;; ANSWER SECTION:
google.nl. 300 IN A 142.251.36.35
;; Query time: 12 msec
;; SERVER: 2a07:a8c1::xx:xxxx#53(2a07:a8c1::xx:xxxx) (TCP)
;; WHEN: Fri Apr 11 14:13:00 CEST 2025
;; MSG SIZE  rcvd: 54

MTR for primary:

2. (waiting for reply)
 3. ae0-1000.core0.freedomnet.nl                                     0.0%   189    4.5  10.5   3.1 117.6  20.7
 4. 2a0d:9c80:1:1:2::30                                              0.0%   189    5.5   4.6   3.0   6.4   0.9
 5. 2a0d:9c80:1:1::46                                                0.0%   189    3.8   5.2   3.1  27.3   3.1
 6. (waiting for reply)
 7. (waiting for reply)
 8. ethernetswp25-ds2-u3-r7-1-68-a.ams1.constant.com                 0.0%   189    6.2   5.4   3.9   7.2   0.9
 9. (waiting for reply)

MTR for secondary:

 2. lo0-3.bras0.freedomnet.nl                 0.0%   150    4.8   4.7   3.1  18.7   2.0
 3. ae0-1000.core0.freedomnet.nl              0.0%   150   19.1  10.7   3.1 120.6  20.8
 4. 2a0d:9c80:1:1:2::30                       0.0%   150    3.5   4.5   3.0   6.4   0.9
 5. 2a0d:9c80:1:1::46                         0.0%   150    5.3   5.3   3.2  26.0   2.7
 6. (waiting for reply)
 7. (waiting for reply)
 8. (waiting for reply)
 9. (waiting for reply)
10. dns2.nextdns.io                           0.0%   150    4.5   5.8   3.7  34.6   3.0

Already contacted my ISP, and they could reproduce the issue, they changed the routing, but it had no impact unfortunately (they could see that as well).

Tried to use the diag tool, but it didn't want to POST since the dns lookup for that failed (commented on an existing issue there).