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It's because updating dns does not work reliably so it's always a lot if trail and error which you can only see after the cache updates

Could you make your changes with a low TTL and switch to a longer one once you are satisfied with the results?

How does that help you if you have to wait for long ttl to expire before the short one takes over?

You don't if you flush your cache... on linux, that usually means a reboot unless you run bind or dnsmasq and can just restart them.