I have looked into this a bit, and so far it looks like this is a bug in iOS. See: 252544 – Initial ServiceWorkerWindowClient in a Home Screen web app launched to handle notificationclick handler is inert for a short period
They key piece there is probably this comment from Brady Eidson (Webkit engineer):
In my test case it’s working because I’m postMessaging to a new client opened via clients.openWindow
If I attempted to matchAll and reuse an existing window client, I would fall into this same trap.
Looks like Brady has a PR up for this as well: https://github.com/WebKit/WebKit/pull/11848, probably best to wait and see if this gets fixed upstream.