As seen in the screenshot, the wooden stick of the banner is rendered in front of the banner pattern on the toast while it is behind the pattern in the advancement widget.
 

This may be a duplicate of the enchantment glint for enchantment books on the toast as the toast appears to have some issue rendering certain items, but I am not sure.

Important note: This texture bug occurs when you open the advancements menu at the same time or during the notification toast.