Thanks for replying and the suggestion, but for some reason it stopped happening shortly after posting, and the issue has not returned, but I'm not exactly sure why. And I cannot make it reoccur.

FWIW, when the issue was present, I had 6 different ost/nst files in my %localappdata%\Microsoft\Outlook folder. A new one would get created every time I launched Outlook at a new login. All had the same filename with the traditional Windows (1), (2), (3) at the end of the filename.