Unwanted overlay atop shortcuts in iOS 18.1.1 (but only when executed from bookmark on Home Screen)

I've only recently started experimenting with shortcuts, and one of the few I've created is to select photos from the Photos app, combine them into a grid, and save the results back to Photos. It worked fine in iOS 17, but since upgrading to iOS 18.1.1 I now see the following when I tap the shortcut's bookmark on the Home Screen (photo thumbnails intentionally blurred out):

The issue is a rounded bar with what looks like a "home" button icon on it near the top of the screen. It blocks access to selections, rendering the shortcut unusable, and can't be swiped away or otherwise dismissed. Tapping the button icon in the center of the bar closes the shortcut and returns to the Home Screen, so it may indeed by a virtual home button. But I have no idea where it came from.

The overlay doesn't appear if the shortcut is executed from the Shortcuts app, only if it's executed from a bookmark on the Home Screen. It also shows up on other shortcuts that require input (e.g. one that places content into an email but needs the user to enter the recipient), but again only when executed from a bookmark.

I suspect I've inadvertently activated something in Accessibility or elsewhere that's responsible, but can't find what. Deleting the shortcut and recreating it in iOS 18.1.1 doesn't help, I get the same behavior.

Anyone know what this is and how to get rid of it?