Hi,
i have the same problem as described here: https://avidemux.org/smif/index.php?topic=18338.0
just with mp4 conainter. The picture is not scaled up so i have black bars around.
Sadly i cannot use an up to date version, i get an error while starting (i open a seperate topic)
Edit: Solved. I was able to install a newer version of 2.6 there is an option to set aspect ratio.
Thanks