Disappearing flight boarding passes in Apple Wallet (not expired!)

I've just come back from a family holiday where at least two (out of five) of us have had issues with future boarding passes disappearing from Apple Wallet.

I have an iPhone XR on iOS 15.6.1. I have used Apple Wallet extensively in the past, and have it set not to hide expired passes.

I had boarding passes for two different airlines - one downloaded directly from their app, one sent to via messages both disappear from wallet (before the date of the flights) after adding them. Upon re adding them, the same behaviour has taken place. they will show up at first, but by the next day they have disappeared.

The exact same thing happened for my partners father on an iPhone 8.

Strangely they did not disappear on some people devices - one where they are definitely still present is an iPhone 12 pro.

Does anyone have any idea what may be causing this?