SPFx app customizer not loading on using browser back button

nayann20 , you may be facing the issue described here: Handle Navigation in SharePoint Framework Application Customizer - Nanddeep Nachan Blogs

The solution would be to add an event handler for the navigatedEvent in your onInit() method like so: 

this.context.application.navigatedEvent.add(this, () => {
    this.loadReactComponent();
});

Let me know if the example in the referred page works for you.

Kind regards, Matthias