Here’s my original bug report from 2 years, 4 months ago, complete with the network and system information that was requested by @noris at the time. There was no resolution. My bug report was split and merged into someone else’s bug report which likewise has never been resolved.
Here are search results containing about 160 reports of the “metadata improver paused” bug: Search results for 'metadata improver' - Roon Labs Community. Search hits include keywords describing the bug such as triangle, halted, circle, paused, update software. The handful marked as resolved were “fixed” by random series of reboots of different system components.
Here are hundreds of bug reports about errors adding music to the library, including keywords like stuck, spinning wheel, never finished: Search results for 'adding music to library' - Roon Labs Community.
No additional information about my current network configuration will get roonlabs off the pot on this. It’s clearly a bug in one small area of code that authenticates with their metadata provider backend, and which does this differently when doing batch updates from how any other component of roon authenticates. It’s probably some duplicate legacy code that hasn’t been maintained.