I used an existing GIF and deleted the background on all layers. I saved it again with a transparent background. The image is 150 layers. It a a growing figure (like an opening flower). From layer 78 tot 150 it goes back to where it started (flower closes again in reverse order).
When playing the animation in Photoshop one layer is selected for each frame in the timeline panel, but when I save it as a GIF (dithered 128) and then view it, layer 1–78 go alright one by one, but when it reverse from 78–150 it keeps all the previous layers visible!