It should make them. Which linux are you using ? You can try running a linux graphics tool such as Gwenview which supports .xcf to generate a cache of thumbnails. \Might not work if you are using a flatpak or snap installation.
...and here - kubuntu 2.0 / Gimp 3.0.4 appimage
top is before running gwenview / bottom is after running gwenview on the \images folder.
...and here - kubuntu 2.0 / Gimp 3.0.4 appimage
top is before running gwenview / bottom is after running gwenview on the \images folder.