Add e-tickets to wallet by opening an email on my iPhone?

I frequently buy train tickets from several different train companies in the UK, and until recently was able to add them to my Apple wallet by clicking on the “add to Apple Wallet” button in the email once I’d bought them. The tickets also come attached as pdfs.

The last month or so, the emails have been coming without the “add to Apple Wallet” button and instead say “Or to add your etickets to Wallet, open this email on your iPhone”, which seems to imply they will add automatically if I open the email on my iPhone. This is not working. I am opening the emails on my iPhone Mail app, scrolling all the way down, leaving it open for a while, clicking on the attachments. In short playing around quite a bit. But the tickets are not showing up in Wallet.

I have an iPhone 13 Pro and my iOS is up to date.

Is this actually a thing now, and if so how can I get the train tickets to show up in my wallet? Thanks for any insight!