If recursive resolvers like Cloudflare, Google, or your ISP stopped caching _num.* records, the servers wouldn't go out of sync or falsely return negative results, since DNS caching and updates are more of a "pull" model than "push" with consensus (so it's just like your computer's L1, L2, L3, memory, and disk caches, but with eviction based on TTL).
So _num records would end up forwarding all the way to the authoritative DNS server for the zone you're trying to access. You'd just have a slower experience than with your normal DNS queries, but you wouldn't observe anything weird.
So _num records would end up forwarding all the way to the authoritative DNS server for the zone you're trying to access. You'd just have a slower experience than with your normal DNS queries, but you wouldn't observe anything weird.