I realise there is a new and improved barcode scanner control, however I am limited to creating the app in question to run on a Windows mini PC running Windows 10, so the app is launched from the PowerApps app. I have added the legacy 'web' barcode scanner to test, and have found the following.
In the designer, the scanner control shows up OK (showing a feed from the front camera on my PC) and testing it works fine.
When I run the app on this other Windows 10 machine, the control doesn't appear at all. The 'camera view' simply doesn't appear, even though I can tell that the camera is active as the light next to the camera lens is on. I have confirmed that PowerApps has permission to use the camera, but the control isn't even visible?