Automatic Pool Covers
Due to the relatively tight fitting nature of automatic pool covers they can allow for a buildup of heat and chlorine in the head space above the pool water and below the underside of the cover.
This area of high temperature and high chlorine can lead to increased water absorption into a vinyl pool liner. A high level of water absorption can cause liner wrinkling and discoloration in the same way that it can occur when an excessive level of free chlorine is maintained in the pool water thereby shortening the life of the liner. In fact, the wrinkling and discoloration can occur at a faster rate in the area between the water surface and cover due to the elevated exposure conditions.
With this in mind it is important that automatic covers be vented or periodically opened to minimize the heat buildup and increased free chlorine conditions. Without taking these steps wrinkling and discoloration of the vinyl pool liner are likely to occur. (Technical Bulletin - Use of Automatic Pool Covers-180522, CGT)