Apple Pay screen appears randomly

Hi,

I've been using Apple Pay for about a year now on my iPhone SE, very happy with the convenience and so on, but here's a strange problem...

Every so often after pressing the "Home" key this screen comes up, as if I'm expected to pay for something. Fortunately it's never happened near a card reader but I'm worried if it did. It doesn't stay long, usually I panic, hit "Home" again and it disappears. Finally managed to take a screen shot today.

...I've touched the photo up to remove my card details, obviously. Notice the border at the bottom of the screen too - that's from the Etihad Airways check in app. I was on that flight, yes, but I deleted the app end of August 2018.

This problem has persisted through at least one OS update and I've tried restoring my iPhone SE completely twice. Still happening.

What can I do? It suggests there's something corrupt in there, which in the case of an app like Apple Pay makes me nervous.

Thanks

ETA - just realised that I can make this screen appear by triple tapping the "Home" button, every time. How can I prevent this?