Why DNS zone changes are not propagated across Google public resolvers? Do I really have to wait 2 days for everything to update?
As with most internet protocols, not everything obeys the DNS specification. Some ISP DNS servers will cache records for longer than the TTL specifies, for instance for 2 days instead of 5 minutes.
In practice, when updating a DNS record with a 5 minute TTL, a large percentage of clients will move over to the new IPs quickly (within 15 minutes), and then there will be some percentage of users that slowly update over the next few days.
If the Google nameservers (126.96.36.199 and 188.8.131.52) do not reflect the changes of your zones (for example after updating an IP address of an A record) you might want to flush Google public DNS cache for that specific record.
Google has published a special tool on https://developers.google.com/speed/public-dns/cache.
If the Cloudflare nameserver (184.108.40.206) doesn't reflect the changes of your zones, you may try to purge Cloudflare public DNS cache for that specific record.
Cloudflare's special tool for purging DNS cache can be accessed at: https://220.127.116.11/purge-cache/
Credit goes to Julia Evans.