29th November 2017, 19:40
In my case, it turned out to be a problem with the screen scaling. I have a high DPI screen and I've used 1.5 scaling factor for the screen. It messed up the underlining of the misspelt words.
I've reported it here: https://bugs.kde.org/show_bug.cgi?id=383922
I've reported it here: https://bugs.kde.org/show_bug.cgi?id=383922