Yes highlighting and heat styling can damage the curl pattern, temporarily or permanently. Work with your hair properties, if your hair is fine or damaged use hydrolysed protein, if it's porous or lacking in elasticity use coconut oil. Also beneficial for damaged hair includes ceramides, 18-MEA and panthenol.

I find my underlayers suffer from drying out too quickly when plopping too.
Originally Posted by Firefox7275

I am beginning to wonder if plopping and I are not destined for each other. It's SO nice to get my hair out of my way when it's that wet (it's currently about two inches from APL) but it *does* make it straighter.

I have been experimenting with gels too. I tried the Fructis Curl Scrunch Gel with Deva ArcAngel and BRHG all mixed together. The Fructis gives me adorable spirals on top and meh on bottom. Maybe it will work better if I don't plop.