This is a glitch. At least it is for me on XB1.

The progress shows when I move out and then back in to the medals page a few times, or at least twice.

Also you don't have to wait for the end of game screen to check medal progress or even change your tracked medal. If you press  <kbd>ESC</kbd> during a match, it brings up the menu options and to the right of it your current tracked medal.

Click on the medal and it will show you your current progress on the medal screen, and allow you to change your tracked medal. If the progress doesn't show, navigate back out and then in again to see if it works.