SwiftKey disabled account sync in its keyboard late last month after a bizarre error caused predictions to be sent to the wrong users. It said an update was in the works to fix it, and now it's rolling out. SwiftKey v184.108.40.206 will re-enable sync on your device, hopefully without all the crossed wires.
SwiftKey, which was recently acquired by Microsoft, was initially vague about the cause of the sync outage. It emerged later that the company manually turned sync off because of the incorrect predictions. In some cases, users were being shown entire phrases or email addresses that were meant for others. That's kind of a big deal, though SwiftKey contends that only happened to a very small number of users.
The new build will begin syncing your account automatically, allowing you to add new words to your dictionary and get personalized suggestions on new devices. The rollout is in process, so you might not be able to see it in the Play Store yet. The new build should be available to everyone soon.