Frustration Level: 🤬🤬
SOLVED: ✅ 

Simple Answer: 
When Google's Firebase Cloud Messaging (FCM) DOCUMENTATION and GEMINI AI tell you that the app notification icons must not be transparent, DON'T BELIEVE THEM!

Technical Answer:
The latest Android implementations turn the logo and notification icons into MONOCHROME (one-color) icons so that all the notification icons look uniform in both light and dark modes. Android does this by painting the icon using a simple (and fast) graphics algorithm. Every pixel with a color (and yes, white is a color) is painted a single color. So, if you have a really cool app logo with a background color, the result will be a WHITE SQUARE in the notification panel – not what you were expecting. And, NO, you can't fix it programmatically.

The FIX:
Make sure your notification logo has a TRANSPARENT background.

Android turns icons with a colored background into A WHITE SQUARE.
Icons with a transparent background are turned into nice, clean icons in the notifications panel.

See the icon in the
notification bar?
Click image to enlarge.

 Direct link to the article: 
https://www.gridq.com/dev-notes/why-are-the-icons-in-my-android-push-notifications-showing-a-white-square-instead-of-the-app-logo

 tags: Android, FCN, Firebase Cloud Notification, app icon, notification icon, Android white square problem