Right, it’s not the sidecars. When metadata is read from an image on import or with “Reread Metadata From File” a seemingly empty comment record is created in the ImageComments db table that causes the incorrect caption. 8.6.0 doesn’t do this.
There is a fix in the development version that sounds similar:
031 ==> 507576 - digiKam User Comment Exif displaying incorrect characters for Nikon images.