When I export an XCF file to a Collection, it doesn’t appear grouped [SOLVED]

When I export an XCF file to a Collection, it doesn’t appear grouped.
When I export an XCF file to a Collection, it doesn’t appear grouped — in fact, it doesn’t appear at all. However, when I export a JPEG or TIFF file, it shows up properly grouped, as shown in the image below.
Can anyone help me with this?
Thank you very much.

XCF is Gimp’s working file format. These are not image files that darktable can understand.

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