I have had this since 18.2 and it is driving me crazy. I can’t believe more people are not complaining about it. Either it only happens in specific configurations or people don’t try to open emails via notifications.
For what it is worth, it is happening to me on an iPhone 13 mini. I have an exchange work account and Gmail account that I use mail for. Issue happens with both inboxes. I have an Apple Watch 10 connected to the phone although I can’t imagine that’s causing the issue.
One weird thing is that it generally only does the freezing at the inbox for the first email notification when there are multiple notifications stacked up.