I'm sorry but the disappearing URL is not our bug. We have no control over this, we only provide the information (port number) in Asset's package manifest.

The only other package with its own admin web interface I could find - Transmission BT client - has the very same problem.

Please complain at Synology.

There are other ways to open Asset web interface, for an example, in Windows 10 Explorer, "Network locations" will list running Asset instances; right click it, choose Properties, click device webpage link from there.