I made an app with WAB and used the dashboard theme.
I chose 3 widgets - layer list, bookmark and info-graphics.
on mobile view - i can only see the first widget as it should be. The other widgets are shown as blank.
you can see the app here:
you can change the size of your browser window to be small like a mobile device (don't need to actually use your mobile).
I tried it on 3 different apps and it happens every time.
p.s - I also tried making the app using the WAB for developers, on my server, but there is no change. (v2.5 and also v2.6)