So, I've had SwiftKey for many, many years and been through a huge number of phones, but never had this issue.
I have a few devices - one of them being an iPhone, the other being a Samsung.
I recently purchased a new Samsung (S9), and it seems although it shows the sync as being connected, it doesn't 'remember' any of the many words I have on there. It's an issue which I've never had.
I've deleted, reinstalled, clicked sync on both sides, ensured it's logged in etc. No luck.
I have a SwiftKey account for my synchronising. In the most recent update of iOS SwiftKey, it no longer seems to be an option, to log-in with a SwiftKey account, but only with Facebook, Microsoft, Google or Apple. Could this be it?
Any idea what it could be?