Whether it is a bug or the designed behaviour is for Apple to call. You can use iTunes Feedback or Apple Bug Reporter to try to let them know about the issue, but it is generally a one way communication process. They don't talk back, but if they decide the issue warrants action it may get rectified in a subsequent release.
I'm not sure quite what you mean by the statement "I prefer to use the Sort Artist field rather than the Artist field". Use it for what? The value that controls sorting is generally the first non-blank value from Sort Album Artist, Album Artist, Sort Artist, Artist. The value that is displayed is Album Artist or Artist. Whether you sort on Sort Artist or Artist (or Sort Album Artist or Album Artist) the order is the same. What is displayed in the Album grid, or the now playing screen or elsewhere is always the primary field, not the sort field, so what exactly is the impact on your library of this change?
If you must have every slot for Sort Artist filled in when looking at the Songs or other list views, which as far as I can tell is the only place where it might matter visually, then click in the empty fields and type in the details. The order won't change if they match the primary values, but iTunes will accept them and not wipe them out as it does when using Get Info.
tt2