I encountered with exactly same issue icon and its not showing we havent used right aspect ratio of icon image.
Link : https://spilymp.github.io/ibo/
You can simply use this open source app to customize an icon for your app download it freely. (This will maintain the aspect ratio of the icon for odoo.
Give this solution vote and mark it as best for this problem #KanakInfosystem #developer
Not sure why it is not coming, just login and out, click the Update Apps List, clear cache, try the above steps and see whether it makes any difference. If it is visible when you go to more info, there is nothing to worry and nothing to do in the code side, it will appear, it has to appear in the main page.
When I changed from developer mode to user mode the icon appeared. I am not too sure if this is the reason or the solution but it works for me.