I could only reproduce it locally with a debug build, with a reproduction rate of about 60% (tried 10 times, failed 6).

Introduced by https://chromium-review.googlesource.com/c/chromium/src/+/1013211

Failure example: https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-rel/7083

Relevant parts of log:
[31222:31222:0419/202153.364821:INFO:CONSOLE(0)] "[SUCCESS] setCustomJepgBadWallpaper", source: chrome-extension://obklkkbkpaoaejdabbfldmcfplpdgolj/_generated_background_page.html (0)
[31222:31222:0419/202153.373603:INFO:CONSOLE(0)] "[SUCCESS] setWallpaperFromFileSystem", source: chrome-extension://obklkkbkpaoaejdabbfldmcfplpdgolj/_generated_background_page.html (0)
[31222:31222:0419/202153.381192:INFO:CONSOLE(0)] "[SUCCESS] getAndSetThumbnail", source: chrome-extension://obklkkbkpaoaejdabbfldmcfplpdgolj/_generated_background_page.html (0)
[31222:31222:0419/202153.388402:INFO:CONSOLE(0)] "[FAIL] getOfflineWallpaperList: API Test Error in getOfflineWallpaperList
Actual: undefined
Expected: test1.jpg