The fallback is just stupid. Make it a hard fail in that case, as you pointed out it is a very uncommon scenario. If you really, really want to have fallback, why not have list of root DNS servers and have the resolver do recursive query. I set up my network that way (don't use google, cloudflare, my ISP or anything else) and used it for years without problem.

But I think hard failure is best approach in this case, because this behavior will also make it harder to troubleshoot issues, let say you are setting up a network and misconfigured your router that doesn't provide DNS now you will be puzzled why Internet works on some machines while it doesn't work on others.