Solved, at least for the Mail app! After updating to IOS 18.3.1 on my iPhone 12 and making sure lock screen notifications were turned on, new mail made a sound, but there was no banner on the lock screen, even after trying all suggestions I found online. I discovered the fix hidden deep in the notification settings for the Mail app. Besides making sure lock screen notifications are turned on, go to Customize Notifications (bottom of Mail Notifications page), and at the bottom of the Customize Notifications page, select Primary Inbox and make sure that Alerts is turned on. If it isn't, you get only audible alerts on the lock screen. Now the banner pops up along with the audible alert.