I have a couple of clients that are having the same issue, but for them, Safari is also not resolving the host name. I tried manually resolving the hostname on the iPhone cellular network DNS servers, and they do resolve it. The conclusion was that it's an iPhone DNS cache that is failing; the mystery is why the iPhone is not fallbacking into the server 🤷🏻‍♂️. Resetting the Network Settings on the iPhone solved the issue.