We found in testing that connections which load a RemoteApp (like Notepad.exe) connect by default using the English (US) language.  According to Microsoft, the reason it happens is that connections are failing to map the input-language settings properly.  This article explains the behavior: https://support.microsoft.com/en-us/help/322042/input-language-of-terminal-server-client-does-not-match-that-of-terminal-server-session .

According to the Microsoft article a simple registry edit on the application servers will allow it to ignores this default remote keyboard behavior:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]

"IgnoreRemoteKeyboardLayout"=dword:00000001  (set the value to 1 essentially)

Doing this will allow webRDP to make a connection.