Let me try to explain. I encountered this in train stations in two countries: Netherlands and France.
These stations have scanners at gate that accept both optical tickets (QR codes) and NFC tickets (like Apple Wallet). Here is a photo of the gates in Netherlands: https://hilario.bambooradical.com/benelux/images/train_02_check_in_gate_close_up.JPG
In both occasions, I had a QR code ticket as a PDF (not in Apple Wallet). When I brought my iPhone screen closer to the reader at the gate, Apple Wallet thought I wanted to pay using one of my Apple Pay cards and automatically brought the Apple Wallet screen. There was no way to keep it off. So the QR code in the PDF wasn't visible long enough for the reader at the gate to recognise it. I tried many many times but nothing helped. In the first instance I almost missed my train!
I already have the double click side button on. It doesn't matter, Apple Wallet automatically pops-up as soon as it detects an NFC reader.
Hope this helps clarify the problem.