Hello,

sorry but i can't help you so much because it's an issue not due directly to Joomla 3.8 or to JSitemap.

What happens is that the iframes are now reloaded to the main window page. Thus the iframe used for the sitemap is reloaded in the main window page.

I guess that you have some third party script responsible of this strange behavior.

I can give you hints. Firstly disable JCH optimize and inspect scripts/plugins to understand which one is doing this reload for iframes.

Regards