Scribble Undo in Neoreader - Why is it so hard?

I used my pen. I scribbled a little. Why can’t I undo the drawing, only by the eraser? I tried the “undo” button (looks line an horizontal scroller with a curved right-to-left arrow above it) both on the floating toolbar and on the bottom menu (to the right of the TTS, slideshow and the page thumbnail view), and it’s not working? Why so hard???