Unfortunately, you can't conclude that you did ANYTHING wrong. I checked this out about 15 years ago, and again this spring.

The bottom line is discouraging: some liner patterns and colors are not very chlorine resistant, but there's NO WAY for consumers (or even pool pros) to tell which is which.

As best I can tell, the more expensive Plastimayd liners are resistant. Many others are as well, but since liner manufactures simply assemble (cut, glue, add bead) liners from purchased rolls of sheeting, until the liner mfg start labeling their liners by sheeting source and sheet model, neither you nor I can tell.

The good news is -- bleaching color and pattern is unlikely to damage the liner itself, just its appearance.

Ben