As a test, delete your own icon and let B4A set the default icon. Is it still missing?
I deleted: Objects\res\drawable\icon.png
I then clicked 'Tools' > 'Clean Project' and complied the app again.
Now the B4A default icon displayed for both my app icon and the icon in the notification.
I then selected my icon which is a png file (512x512 image), then cleaned projected again. My app icon is the correct icon again, but the notification icon is blank again.
I then checked the default one that b4A uses and it's a 128x128 png image. I then re-sized my icon to be 128x128 and I got the same result. App icon is fine but the notification icon is blank.
I then created a new image (512x512) made it so that the image has a round circle.
I then copied the image to Objects\res\drawable and named the image to 'notificationicon.png' and made it a readonly image.
In my code I changed
n.Icon = "icon" to
n.Icon = "notificationicon"
When I run my app, I am getting my normal app icon, which is what I want. Then when the notification comes through the icon is a round circle. For some reason the icon is not in colour. I guess my original app icon would of worked (or is working) except since it is showing as one colour then it making it look like it's blank (all grey). My test app showed my notification icon in colour though.