The adding new words has been through the Preferences dialog.
I clicked new, typed in the word and pressed enter. Repeat three times, then close the preferences window. They are all in default. Open preferences window, type in a new word, press enter, close the dialog box, then the new word wipes out the three previous words in default. When you close the dialog box then reopen, no list of words is visible.
What is interesting, is that it did not work this way until this morning. I could not enter new words through the preference dialog, it would not take them. It would not switch to entry mode so they could be typed in.
Will try this a bit later. It is already about 9 pm where you are even if it is just about 1 pm here.