(non-woven) wallpaper can shrink slightly due to temperature differences. If you have a dark (non-woven) wallpaper on a white surface, the seams will immediately stand out.
How do I solve it?
There is no real solution to this problem. You have to prevent it. In the first place, avoid large temperature differences before and after assembling. Also avoid drafts – for example through an open window – during assembling. Another tip to avoid open seams: first place a primer in the color of the wallpaper. In case the wallpaper shrinks a little bit, you will not notice the seams as much.