I have noticed that IE flags the layers coming in from non https sources as "Risky Content" forcing users to click "allow content" at the bottom of the webpage.
I see that this only happens in IE when the AGO app has "mixed" sources (mixed= http & https) as I have apps that only pull in http sources and I do NOT generate the IE warning message.
This only occurs in IE.
If users do not click "Allow Content" then the map loads as if it cannot find the http source
It seems like that's how IE works...when it encounters the https layer, it switches to all https mode. A potential easy fix would be to add the National Floods Hazard layer via http instead of https, if possible.