When images are uploaded in WordPress 4.6.1 that have an empty alt text value, WordPress tries to generate one based on the caption text or the image title. If the image title is non descriptive as is common with photos uploaded from digital cameras, the alt text can be meaningless.
In WordPress 4.7, the caption text and the image title fallbacks have been removed. The fallbacks were originally introduced to ensure every image included alternative text. Over time however, this practice has proven to be a poor user experience for people who use screen readers.
Since the fallbacks are removed, users will need to explicitly set a value for the alt text field. According to Joe McGill, the change will not affect content already published but will be the expected behavior in WordPress 4.7 and beyond.
If you’re not sure what text to use to describe an image, check out this article on Webaim. It explains when alt text should be displayed and provides useful tips on how to describe an image.