If it helps I have a script that I run to check which sites have sitemaps that are incorrect. I haven't spent much time on the issue but another xml sitemap plugin had run into this issue and fixed it - commercial one though and the fix was never posted on their support channel.
The script - run at your own risk but I've never had an issues with it. https://www.dropbox.com/s/t38odjjcx0f2ijw/checksitemaps.txt?dl=0