Since 28 April, we have seen some bizarre behaviour where iOS 26.3 and 26.4 are intermittently not loading some CSS and JS resources from our CDN. This is only reproducible when our CDN has HTTP/3 enabled.
When reproduced in the Simulator, Safari's HAR shows that it is not even attempting to request those resources; it does not appear to be a network issue. Oddly enough switching to a different CDN with HTTP/3 enabled appears to resolve the issue.
As far as I can tell, this hasn't been reported on the Webkit tracker; we'd be happy to provide Apple with additional data in a formal bug report but it would be helpful to know what data would be useful to provide.