A side brush is a common part of most robot vacuum cleaners. Usually, a side brush last 3-12 months of regular usage before you need to replace it. The bush tends to lose its shape over time. Some YouTuber posted a video on how to restore a deformed side brush. I find it very helpful, so decided to share the video with you.
Not sure if this will work with all types of side brushes, but if your brush looks similar to the one used in the video, I guess it should work. Let me know in the comments if you was able to restore the brush using this method.