I've had the same problem. My SSD is pretty full, so I deleted the game (backed up local save files, not cloud saves), and reinstalled from scratch to get the patch. Then I saw that the Editor and Data files were free on Steam, and that the Editor was "free to play" and in my library, but that there was still a download button to get the data files. I downloaded the Editor Data files, made the editor visible in my library per Larian's instructions, then booted the editor up to see what it was like. In order to use the editor, you have to start a project. I had to choose an existing level in the game to edit, presumably from the 10+ GB data files. I just peeked at what I loaded up in the editor, but didn't really mess with anything (it's beyond my technical skill). I think it saved a "project file".
Then I loaded up BG3 and got the "Data Mismatch" message, but have had no problems running the game, or using my preexisting local save games. Verified the files and they are all good. Loaded a few new mods (bangsx3 and some dice)...no problems. It's just that the damn mismatch message won't go away.
I think because the editor created a project, the game notices it, even though it's doing nothing, and flags it as a data mismatch. Of course, I'm just guessing here...
