This is expected since SSE as a technology "long poll"s with the server to receive information back from the server as a stream of events. These requests are still standard HTTP requests but instead of ending the request and getting the response as quickly as possible, it keeps the connection open waiting for event messages to be pushed from the server.
Please post the errors you are seeing from your Safari even if you don’t think they provide much useful information, it might provide additional context to your issue to help find the root cause.
It is also worth noting, if you are using some kind of reverse proxy between the client and server, this can cause network issues, is the iPad on a different network? I’ve seen these kinds of issues in corporate shared networks where their internet access runs through security proxies setup in the corporate environment that can interfere with SSE sending information back to the client. See this thread as an example of troubleshooting with a reverse proxy in the mix.
Without error information, we can only guess. Network information about the failed requests will likely provide a lot more context to help with trying to identify the root cause. Between days, did the browser remain open? Was the computer put to sleep between working and not working? Did a reload of the page fix it? Are you seeing any failed heartbeat requests?
You would have to provide an example of how your clients are connecting, and related errors you are seeing to rule it out, but generally I wouldn’t think so.