Using Chrome in Windows, with several new, unlinked images uploaded from the app, select an image to transcribe. Click "Link more images". Click the plus sign on a related image that has not been transcribed (image with a grey background not blue). The website pops up an alert box that says the images have already been transcribed and asks if you are sure you want to link them. This alert box should only pop up for transcribed images with blue background, not untranscribed images.
This bug has been in the Chrome version for ages (at least a year) and is rather annoying, but I have learned to ignore it. Doesn't really make any difference operationally, at least to me.