Over the weekend, some Facebook users discovered that not only did the company force new @facebook.com email addresses on them earlier this week, they also got hit with having Facebook change and overwrite their contacts' email addresses to the new @facebook.com address. This caused some Facebook users to lose some emails.
Now, according to an article on The Verge, Facebook says that the overwriting of the email contacts was in fact a contact sync API bug. The issue just happened to occur around the same time Facebook decided to make its @Facebook.com email changes.
According to a statement from Facebook:
Contact synchronization on devices is performed through an API. For most devices, we've verified that the API is working correctly and pulling the primary email address associated with the users' Facebook account. However, for people on certain devices, a bug meant that the device was pulling the last email address added to the account rather than the primary email address, resulting in @facebook.com addresses being pulled. We are in the process of fixing this issue and it will be resolved soon. After that, those specific devices should pull the correct addresses.
While this bug will fix the problem with some users' contacts being rewritten, there's still no change to Facebook's original decision to make the @facebook.com address the only viable one on your account, unless you make the effort to make your other email addresses visible.
Source: The Verge