Randomly the Auto Spell Checking stopped working: Wrong spelled words are not underlined. Deactivate and reactivate it and it works again. (With the right click option) Actually I think the spell checking works fine and just the underlining fails. Because right click on a misspelled word still recommends other words. Reproducible: Sometimes Steps to Reproduce: 1. Type 7-9 misspelled words. Actual Results: The red underlining is gone. Expected Results: The misspelled words have a red underlining. I just tested this with the german dictionary.
KDE's spell checking by default stops underlining misspelled words if there are (a) more than ten words in total, (b) more than 42% of them are misspelled or (c) more than 100 misspelled words have been detected. The only recourse is to disable and reenable. The idea is presumably not to underline every word on text in the wrong language. We can do nothing about that. You could play around with adding the Sonnet_AsYouTypeDisablePercentage and Sonnet_AsYouTypeDisableWordCount keys to your sonnetrc config file and changing the values from the default 42 and 100.
(Addendum: In the [Spelling] group.)