Some titles identify under Artur, some under Arthur. Bio info shows under Artur but not under Arthur… haven’t got Roon to identify and provide info for titles in the 81 Disc Rubinstein Collection series yet either.
I also have the complete Rubinstein, and Roon has done pretty well with it.
Allmusic has him as Artur, so you would be well advised to run all your files through an external tag editor and make sure Album Artist/Track Artist is always Artur Rubinstein. That would give Roon a head start. Beyond that, you can manually correct most of them.
I have all but about 10 albums from the set recognised without any massaging.
There are one or two which are unfortunately not in Allmusic with full metadata. But it’s only a few.