This is actually something we directly control!Is this likely to be a Google issue which you have not much control over?

The passes are grouped by an arbitrary identifier which we provide, and we coincidentally did some work to improve how these are generated which went live on 9 October, which was after you made your booking.
I think this shouldn't happen again, but I'll reproduce your purchase in one of our development environments to double-check and pass it on to the person who generates the grouping identifiers for my Google Wallet implementation if it's still an issue.