@Saurabh @[deleted] Seems like a serious issue to me as well.
Now, lemme explain you what is happening from my side.
Store App Installed.
App Auto-Start DONE.
Remove from Battery Optimization DONE.
(Anyone not doing all these steps could face the problem anyway.)
Even after these steps, sometimes, the app is going into sleep mode and not delivering notification. And this is faced by all users, Delivery App and Customer App included. One of the major issue is also that few people keep their media volume mute which result in missing notification. You need to set it to notification or ringer tone and not media tone.
Rest of all, please be calm as a fix is on it's way.