In the third released developer preview version of Android 12, it turns out that the notification design has been overhauled and minor changes have been made.
In almost every Android version, some innovations are made in notifications, even if there is no full-fledged change. Therefore, this continued in the latest Android 12 DP3 update.
The redesign of notification cards does not include any major changes, except for the repositioning of the number of notifications for stacked cards. For example, on Android 11, if you have more than one notification from a particular app, a counter appears in the lower right corner of the stacked cards that contains the number of notifications. On Android 12, this counter is positioned right next to the expansion arrow in the upper right corner. Also, the number of notifications, the background is highlighted in color.
This change not only improves visibility but also saves a little more space for text preview on the card. But as is often the case with developer previews, this design may also change in the final version.