Look at the linked URL, and see the resource loading order. The launch of prefetch requests is occurring too early. Prefetch requests should not be launched while, for instance, the onload is pending.
This issue still applies in iOS 13.5.1 and Safari 13.1 on mojave
It's particularly bad if the HTTP/2 server doesn't support effective prioritisation as Safari makes the requests at a low priority but the server ignores this and responds, potentially blocking higher priority responses.