To understand why the pattern may flake off, there must be a basic understanding of the procedures involved in the printing of your pool liner. The print pattern is applied by a process called "roto-gravure" printing.
The inks used are solvent based and when applied to the vinyl they actually bond themselves to the vinyl by "biting" into it. Then a clear "top coat" is applied to increase abrasion resistance and provide an added layer of UV protection.
The cause of ink flaking off the vinyl is low water pH. An acidic environment will weaken the bond by softening the coating and eventually the ink. The more acidic the environment the greater the likelihood of damage. The effect is cumulative and irreversible. Once this softening occurs, the coating and ink are susceptible to abrasion and flaking.
Always keep your pool at the recommended pH of 7.2 to 7.6. Deviation from these levels will adversely affect the performance of your liner.