Why does the time not always display in the top mini status bar of my iphone screen

The clock in the status bar will be hidden in some apps. Just as you also do not see the clock in the status bar on the Lock Screen, as the large clock is already displayed.

If you use an iPhone with FaceID - then you may experience that the clock in the upper right corner is replaced with other information for a short moment, and then comes back. This is because there is less space in the status bar, so the clock can be dynamic.

It is the developer of the app you are using who decides whether the clock should be displayed. Some apps, for example in Games, it might spoil the experience if there is a clock in the interface.

It never hurts to send Apple a Feedback, you can use the link that imobi wrote in his reply.

Apple provides a method for you to leave constructive suggestions, feature requests, and other non-rant comments at Product Feedback - Apple