Swiftkey backup & sync not working

I am using Swiftkey 7.4.0.18 on Android 9 (before that, on Android 7). I got a new phone and installed Swiftkey on it, logged in and enabled backup & sync. My personal dictionary had been correctly backed up before, I can see that in the "View and manage your data" section. The dictionary contains several hundred or thousand words (I speak a special German dialect, that's why). However, these learned words do not show up on my new phone, even though sync appears to "work" as there is no error message. I have contacted Swiftkey support a while ago but haven't heard anything. Anybody here who can tell me how I can debug / solve this?