currently credits all page metrics to the exact URL that the user is referred to by Google Search. Although this provides very specific data, it makes property management more difficult; for example: if your site has mobile and desktop versions on different properties, you must open multiple properties to see all your Search data for the same piece of content.
For users with separate mobile or AMP pages, it unifies all (or most, since some mobile URLs may end up as canonical) of your data to a single property (the "canonical" property).
At the property level, you will see data from your alternate property (for example, your mobile site) shifted to your "canonical property". Your alternate property traffic probably won't drop to zero in Search Console because canonicalization is at the page, not the property level, and your mobile property might have some canonical pages. However, for most users, most property-level data will shift to one property. AMP property traffic will drop to zero in most cases (except for self-canonical pages).
If you want to save your traffic data calculated using the current system, you should download your data using either the Performance report's Export Data button, or using the Search Console API.
We know that this change might seem a little confusing at first, but we're confident that it will simplify your job of tracking traffic data for your site. If you have any questions or concerns, please reach out on the Webmaster Help Forum.