With the new version of Gimp (2.9 ff) the XCF file format has changed. Therefore the XCF previews/thumbnails in dolphin do not show the image anymore for XCFs saved with a newer version than 2.8. Dolphin only shows the Gimp mimetype icon as icon and as preview (sidebar, pop-up). Any chance to get the preview for XCF back? Not as substitute for the old preview format but as an addition because the old XCF format is still possible and will still be around for - years I guess.
If you can provide the information requested in bug 360821 comment #2, please add it there. *** This bug has been marked as a duplicate of bug 360821 ***