The thumbnail does not show in spotlight search results on iOS15 after setting thumbnailData or thumbnailURL in CSSearchableItemAttributeSet and assigning it to NSUserActivity.
I have tried setting UIImage.pngData() to thumbnailData and saving an image to a local file URL and setting it to thumbnailURL. I have tried on physical device running iOS 15.2 and 15.3, and simulator running iOS 15.3.
See attached code. It works on simulator running iOS 14.5 https://www.dropbox.com/s/odax9d3s6htl0fb/SpotlightTest.zip?dl=0
Feedback ticket: FB9915366 (Spotlight Search does not show thumbnail for NSUserActivity and CSSearchabltItem)