putting timeline layers in a group - scrubbing playhead - animation stock-still on canvas

[Wolfering High]

I organized some layers in a timeline-folder. These layers contain grafic-symbols that are (is that the right word?:) movie-tweens. Since I organized them in one folder I can’t see the animation on the stage by scrubbing the playhead. When I take the layers out of the folder they still don’t show action on the stage. Just testing it in the browser makes the animation visible.

Is there a way to prevent and to undo this?

I'm using Animate CC 2019 on iMac with High-Sierra.