With my team we developed a Hub Site and a WAB 3D app.
The problem is that recetly we saw that the embedded WAB3D app is not showing in our hub. Instead it shows a gray space.
Logging the site it shows 404 errors when it tries to load the widgets.
Is there any issue in hub or WAB that doesn't allow us to embed our app?
I'm not seeing this across the board with WAB. Something seems unhappy about this particular WAB as it does this regardless of embed. I'll flag someone from the WAB team to take a look - ArcGIS Web Application
Looks like this is a bug in the "App Card"
While we work on a fix, I suggest you use an "iframe" card and paste the URL you provided there. We'll work on getting a fix out for the app card.