What methods use to determine or a timer to fall back really doesn't matter..
At some point your going to have a bad experience if you point a client to more than 1 NS that can respond differently.. Be that with not knowing anything about what your asking about, a local domain for example or if there is any filtering being done or not, etc.. where your answer might or might not be filtered depending on what NS you asked.
If your going to point your client to more than 1 NS they should answer the same.. Any of them should be able to resolve your local domain, or if external they should all either filter (the same) or not filter..
This really isn't anything new, this has been how it as been since like dns has even been a thing..
And yeah still to this day I see admin's configuring their dns for a client that could be problematic.. They might get lucky, they might not - but its bad practice in my 30 some years experience in the biz to point to multiple NS that might respond not exactly how you want.. if you ask google dns for something host.home.arpa for example - its going to return NX, and when it does your client won't ask anyone else..
; <<>> DiG 9.16.45 <<>> @8.8.8.8 www.home.arpa
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 26063
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;www.home.arpa. IN A
;; AUTHORITY SECTION:
home.arpa. 1800 IN SOA prisoner.iana.org. hostmaster.root-servers.org. 1 604800 60 604800 604800
;; Query time: 77 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Wed Jan 03 15:36:50 Central Standard Time 2024
;; MSG SIZE rcvd: 119
And or maybe it returns the wrong IP because you were using a public domain locally.. And if suppose to be filtered, and asked 8.8.8.8 vs 1.1.1.1 for example and its not - your going to have a bad day.. Can be even worse if your forwarding from some common local dns that multiples use, and it happens to query some upstream NS that doesn't filter, and now all your clients can access this site because your local NS cached it.