Update: searching for a recently added album has made the whole library load properly. Looks like a caching issue? (Running Roon Server on a Mac and Arc on an iPhone in this instance.)
Good that it came right, and thanks for coming back to say what fixed it. That detail is the useful part: a search for a recently added album forcing the rest of the library to load tells us something specific about where it stalled.
So please keep an eye on it, and if it happens again, please note roughly when and whether the same trick clears it. With a timestamp we can pull the server side and see what Roon ARC actually asked for at that moment. If it does not come back, there is nothing here to chase.