Resources
What is DNS propagation?
Propagation is the wait between a change on the primary and the moment every resolver in the world sees it.
Nothing is actually pushed to resolvers. They keep the old answer until its TTL expires and then ask again — which is why the TTL, not the change, sets the pace.
Between our own nameservers there is no waiting: the primary sends a NOTIFY and each secondary transfers the zone immediately.
To check a change, ask a secondary directly rather than a public resolver. A public resolver may still be inside its cache window.
If a secondary answers an old value, compare the serial first. A serial that did not rise means the change never travelled.