Not resolved to my satisfaction. I still use the apple-touch-startup-image link method. It does still work for me but it involves specifying every i-device resolution and is a complete pain.
If it is not working for you, double check that you have an exact match for your device's screen resolution in your list of apple-touch-startup-image links.
I got the method from here: https://medium.com/appscope/adding-custom-ios-splash-screens-to-your-progressive-web-app-41a9b18bdca3
and the sizes from here: https://www.ios-resolution.com