So, Mr.
Desiderius .
I had these annoying "unknown key" entries on most of my popups, usually 2 of them: up under the title and down at the end of the popup.
I uninstalled the game through Steam and downloaded it again. It did not help, at least with my old saves.
Then I applied the latest patch, deleted all the saves and created a new character using the latest version of the game. The only modification I use is custom portraits.
Now I have consistent "unknown key" message under all spell titles - and nowhere else. It seems (though I am not sure) that the "unknown key" at the bottom changed to the place where the item was acquired. Unfortunately, I do not know what exactly changed this: the new patch, the new save or both. But it is better now.
This is the beta for you. They definitely do not bother with save and patch compatibility - which is to be expected. They have more important issues to fix and features to add. If the same behavior with patches continues after the release - then Owlcat Games developers deserve to live on the streets of Moscow in -30 Celsius, but not before.