Adobe after effects undo all my edit,why???

Rotobrush must be frozen to save the changes. If you did not freeze, it has to propagate again. Having to start over is not a bug. It is the way the tool was designed. 

If you spend more than a few minutes using Rotobrush, it is a very good idea to pre-compose the layer when Rotobrush has completed and then frozen, then go to the Composition menu and select Prerender. Follow the prompts to choose a location and then render the file. This will create a production-quality video with an alpha channel of your work. Rotobrush is a finicky resource-intensive tool that bloats the project file and can cause problems. Unless I can do the rotoscoping in about 5 minutes, I always, and I mean always, pre-render the layer and replace it in the composition. I then delete the Rotobrush comp completely.