Google Apps gmail not always in All Inboxes

In addition to my iCloud account, I have a gmail account via Google Apps from my old job (a prestigious university account that I can use for life). Both are working fine on my iPhone 13 mini iOS 17.2, but I’ve discovered that the gmail emails aren’t always showing up in the “All Inboxes” list.

When I did a search for a missing email message (search done on the iPhone Home Screen), I got the message that Siri found the email in, it said, Google Apps gmail. I clicked on that search result and was taken to the “[School name] Google Apps” inbox where I discovered many more unread emails.

This is not related to an iOS update since this has apparently been happening for some time.

Questions: why does this happen? If Siri could find it, why didn’t Mail also find it and put it in All Inboxes? Finally, is there any way to fix this issue?