I have noticed that when viewing a feature layer's item page on a mobile device and going to visualization is resulting in the embedded map displaying a 404 error. I have noticed this on two of my instances of ArcGIS Enterprise. After digging into the Chrome developer tools, it looks like it is resulting in a 404 error when trying to access /apps/preplanned/mobilemap.html. Not an issue on a laptop since it doesn't load the mobilemap.html on that screen size.
I was able to verify it was not an issue with the web adaptor on our web servers since it also happens when going through port 7443. The issue is that it is leaving off the web context of /arcgis or /portal at the beginning of the request for the mobilemap.html.
I have been able to find a handful of other 10.8 portals (5 different organizations) that are showing the same behavior, so I am leaning toward this being a bug.
I do have a ticket open with tech support, but thought I would post here to see if anyone else has noticed this or if it is not an issue in their implementation.