I'm wondering if your Cokin filters may have been exposed to heat to cause the distortion.

I've never used Cokin filters, but in the 1980s, Ambico made a competing product, and I went with the Ambico filters because they were 3x3" and not a proprietary size, so I could use the Ambico bellows shade either with their filters or gels or any other 3x3" filters. They were in the same price range, and are made of the same CR39 resin. I have many of them to this day, and they seem fine. I've tested them against various glass filters for flare and ghosting, and they aren't as good as multicoated filters, but they are as good in this regard as single coated filters and better than uncoated filters.