Just that. I selected a .jpg, for which, "The query did not return any results.", doing a duplicates search. However, when I next, do a search for "Visually similar images...", there are clear matches (both - the same and different dimensions) being found! So, what's causing such inaccuracies?
1. Duplicates searches for images with exactly the same image data. This search mode is tolerant against changes in the metadata.
2. Copies searches for binary identical files.
3. Visually similar images searches for visually similar files, with a lot of fuzzyness and tolerance against editing, slight cropping etc.
From your description I guess that the image data was changed somehow and hence will be neither be found by the Duplicate or Copy search.
The image might look the same to you, but it is not.