Most strange. I do not know what operating system you are using, I am using Windows so the following is based on Windows but the same principle applies to other operating systems, I just dont know where stuff is stored on them
The only thing that springs to mind now is to try a reset of DT and see if the problem is still there. If so it may be a bug somewhere in DT.
First make sure you have the latest installed 3.0.2
Now DT stores all of its info about your images etc in a set of files all stored in the same directory this is in (Windows OS)
Appdata is a hidden folder so you need to change your view options to see this folder.
There are several files there. Create a brand new directory and MOVE (not copy) all of the files to this new directory. This is your backup so that you can restore them after the test
So now the darktable directory is empty (apart from your new directory). So restart DT. It will detect that the files are missing and create a brand new set, all empty and with all the defaults set on
Now repeat the test we did with an import and export untouched, then add the levels and export again?
Did it work? If so there is a setting somewhere you have set inadvertently that is causing the problem
If not then there is another problem and at the moment I have no idea what
Now to restore you database (if you want to)
Go back to the \users\username\AppData\local\darktable directory, delete all the files (but NOT your directory)
Now copy the files out of your save directory back into the darktable directory and you will be back as you were.
Hopefully the reset will fix it, if not I am out of ideas at the moment. Also I am going offline now so wont be back till tomorrow morning some time (UK time)
Incidentally, if you use DT in earnest always back up this directory as well as your images for safety
Good luck and let me know how it goes