The header appears to be fixed in the public release iOS 18.2 with Xcode 16.2.

FB16114803

I filed too, with a sample project. Mine is FB20069803. Note that this occurs on Seed 9, but that's not an option in the "which build" dropdown in Feedback Assistant.

This is slightly different on iOS 26.1 beta 4. The initial page load still displays the same issue, but if you tap a link somewhere on the page and navigate, the blur updates to extend down to the bottom of the bar view.

FB20756572