Excel viewer keeps Personal.XLSB locked

Directory Opus info:
Directory Opus Pro 12.26.3 (Beta) Build 8094 x64
OS 10.0 (B:19043 P:2 T:1) SP 0.0

When an Excel spreadsheet file is highlighted in the details file view and the Viewer Pane is activated to see what is in the file, then the Viewer Pane is closed and the file name is double-clicked to load the file into Excel, Excel responds with "PERSONAL.XLSB is locked for editing by ."

The only way to clear the error, for any subsequent spreadsheet when the Viewer Pane is still closed, is to restart Directory Opus.

We didn't write the Excel viewer, Microsoft did. You'll probably see the same thing if you view the file using File Explorer's viewer pane, as it'll use the same viewer which is part of Office.