Icons in the Web App Manifest are finally supported in Safari on iOS 15.4 beta 1, so you don’t have to provide them via the proprietary way. For some reason, in my demo https://ios-a2hs-demo.glitch.me, it chooses the 32 pixels variant, despite many better options.
<rdar://problem/88276745>
It displays the 120px variant for a split second when the Add to Home Screen dialog shows up, but then switches it for the 32px variant, which it also ends up using on the home screen.
The cause of this bug was outside of WebKit, so resolving as MOVED. The underlying issue was fixed in iOS 16.4.1 and macOS 13.3.1.