I’m also seeing this, and I have the impression that it happens more often since I upgraded to iOS 12 (on an iPad Air first generation).
We have moved your report as to better assist you directly here. Can you please provide an overview of your setup using this thread as a guide?
Please be sure to let me know your Core’s manufacturer/model and the manufacturer/model of your Router, any switches, range extenders, ect.
Are you by any chance using a VPN anywhere in your setup here? Is Roon on the list of allowed applications for your firewall? Please let me know when possible.