I had the same problem as many of you, This was the response I just received from my bank. It’s not a solution but I hope it’s helpful.
From Bank:
I am sorry to hear that the St George Bank Widget disappearing after updating your iPhone to IOS18 has been a poor experience for you.
Customers who have a quick balance widget set up and upgrade to iOS18, no longer have access to their widgets as the widgets are not retained upon upgrade to the new iOS.
Customers who have upgraded to iOS18 and wish to create a widget, do not have the ability to do so.
Unfortunately the quick balance widget is not supported for customers using iOS18 nor is there a workaround to reinstate customers existing balance widgets.
Customers can still access quick balance as an alternative option via the bank mobile banking app.
We acknowledge that this outcome isn't what you were seeking, and I apologise that we aren't able to accommodate your request.