This was happening for me on iOS 16 as well. It seems to crop back up everytime Gmail updates.
To fix, I went into Notification settings for Gmail in iOS Settings and toggled "Show previews on lockscreen" to some other setting and back to the desired one (in your case, "Always"). Then reboot the phone and it should be fixed.
Started happening after an update to the Gmail app so I assume it's not OS-specific.