There's a known issue when Preferences / Folders / Folder Behaviour / Use 'descript.ion' file comments system instead of NTFS comments is turned on (it's off by default).
MSI files have embedded descriptions, which are what Opus always shows currently, even if a descript.ion file exists. When descript.ion files are enabled, setting the description for an MSI file just updates the descrip.ion file, which is then ignored when determining the description to display. When descript.ion files are disabled (the default), setting the description for an MSI file will edit the MSI file itself and modify the embedded description, which works.