While replaying through the campaign mode, I reached mission 6 when I was met with a very strange bug.

When I tap "OK" it remained at the Briefing screen and only allowed me to
switch between Statistics and Briefing. However for some strange reason
I was able to select and move units while still at this screen.
The solution to fix this on my end was to force quit the mission and restart it again. I have yet to encounter this bug again.