Two likely causes for your recent episode. One would be that the mildew was present beforehand. Second would be that the paper or paste was contaminated when it arrived. The first is the most likely. When I have a major mildew problem in a bath I try to steer the customer away from paper and into a mildew resistant paint. If they really want paper, I'll add a mildewicide to the sizing and use Romans Kitchen and bath wallpaper adhesive under the paper even if it's a prepasted paper. However, even with all that, I don't expect that the customer will be mildew free for more than 3 or 4 years.