This is an upstream bug. I'm just marking it here so people who are interested can keep track of it.
Compare the following two images:
https://commons.wikimedia.org/wiki/File:%22What_a_Brilliant_Idea!%22_Barnstar.png
https://commons.wikimedia.org/wiki/File:%22What_a_Brilliant_Idea!%22_Barnstar.xcf
The first is very close to the PNG that Gimp produces from the XCF file. Thumbnails on the XCF file page should look like the PNG upload's file page.
Version: unspecified
Severity: enhancement