Gmail App Expanded Email Notification Not Collapsed Automatically
This is standard behaviour, however I think it may have been different in older versions of Android/different ROM's.

On all the apps on my device that use expanded notifications, only one seems to be expanded by default. There is (most likely) an option for developers to make it expanded by default, and this may have been recently disabled in the gmail application.

As this is done at a program level, there would be no way to change it.