When I was looking at Cat Bordhi’s website, I came across this. I tend to do it and never thought about the fix being so easy!
“The cure for uneven stockinette!
Many knitters purl more loosely than they knit. This results in uneven stockinette. But with Denise needles, all you have to do is use a smaller tip on the purl side of the circular needle. Do some experimenting, and find out if you need to go down 1, 2, or even 3 sizes. It will vary depending on the yarn and gauge.”