This behavior is probably due to having a proxy configured in the Farm Search Administration dashboard ([CentralAdmin]/searchfarmdashboard.aspx) and the proxies not knowing how to route non Internet requests. You need to let SharePoint search know to not use the proxies for the AppWeb URLs, by adding a wildcard entry in the URL exceptions list
Check your ULS logs on the crawl server for errors with eventID du54.
Update "Do not use proxy server for addresses beginning with:" to add http://[AppPrefix]* (and/or https://[AppPrefix]* if you use SSL sites).
The * must be present as AppWeb URLs are dynamically generated with each new app instance. (I wonder if this is one of the reasons MSFT even have an AppWeb prefix that needs to be defined?).
Note: You can determine the [AppPrefix] from Central Admin