After the latest app updates, in most cases after the upload finishes, there are images left not uploaded.
These were not captured after the upload started, no capture happened during the upload.
It is usually somewhere between 1 and 40 images. Repeated upload attempts will usually get rid of them, eventually - although right now I have 12 images that are uploaded and uploaded, but never disappear from the phone/app. It is not clear whether they actually get uploaded to Mapillary systems for every upload attempt.