Animating Gradient Fills not showing

Dear all,

I am following the Adobe Animate Classroom in a Book, LESSON 9 Animating Shapes and Using Masks.

In the subchapter Animating Color > Adjusting the gradient fills, in page 384 the book says to move the playhead along the timeline between the first and second keyframe.

What happens for me is that the flame disappears between those keyframes either in symbol-editing mode or in the ordinary Timeline. 

What am I missing?

I'm on macOS 12.6.8, Animate 23.0.2

Thank you!