Why didn't the video prompt Xiaomi mobile phone after WeChat locked the screen?

It has nothing to do with the model of your mobile phone. It should be that the "Receive voice and video call invitation reminder" setting is not enabled on your WeChat. The way to open it is as follows:

My mobile phone Android system, WeChat version: 8.0.32

Step 1: Open WeChat and click "I" in the lower right corner.

Step 2, find "Settings" on my page.

Step 3, find "New Message Notification" on the settings page.

Step 4, find "Receive voice and video call invitation reminder" on the new message notification page and open its settings. (green means open)