In Samsung Keyboard, I have Russian layout. In the upper-right corners of buttons "е" and "ь" there are slightly smaller letters "ё" and "ъ", respectively. They are visible even if the option "Layout / Alternative characters" is off. If I tap the Shift (⇧) key, all the letters on keyboard change to uppercase, includjng "Ё" and "Ъ".

The problem is, that in "Keys Cafe", I cannot add additional letters to buttons that act as described above. For example, I tried to add the same letter "ё" to the right of the existing one. The letter disappears when the option "Layout / Alternative characters" is off and is not affected by the Shift key.