android 13 and above have stricter rules for setting custom notification icon

these are few rules for notification icon

• Transparent Background: Icons should not include a fully transparent background to avoid blending with the notification shade.
• Monochrome: Notification icons should ideally be in a single color (usually white), with transparency to create the desired shapes.
• Simplified Shapes: Icons should be simple and easily recognizable at small sizes.

refer the following links

link1

link2