STEPS TO REPRODUCE 1. In Settings, enable "Automatic spell checking by default". 2. Create a new file. Type an incorrect text like "adfsllsdkf" and check that the incorrect text is underlined. 3. Go to "Tools > Highlighting > Other" and enable Diff. 4. Check that the incorrect text like "adfsllsdkf" is still underlined. 5. Save the file as "1.txt". Close the file in Kate. 6. Open the file again. Check that no word is underlined, like if the spell checking didn't work...