This looks like a known bug that was supposed to be fixed. Check that you are using the latest version (0.6.17).
Then please remove the file ~/.local/share/okboard/en-user.txt (but keep a backup copy).
Restart the keyboard (for exemple uncheck then check the "enable okboard" checkbox in OKboard settings application).
And tell me if it solves the issue. If it does, send me above backup file if this is acceptable for you (this file contains all new words you have added to the dictionary).
And do not expect quick fixes at this period of the year