There is a similar issue report on the Microsoft Community site, and some replies said that it might be the WeChat app that takes the Ctrl-A shortcut. But in my case it is the NetEase Mail Master. To fix this, simply go to Mail Master‘s settings and change or delete the screenshot shortcut. See the following screenshot for detail.

[Trouble shooting] Alt-A shortcut taken by another app and conflict with Emacs' M-a