The default configuration of some codex has 'Pixel Aspect Ratio' set to 1:1 (fe. MPEG-4 AVC).
Many my files were compatible with this and I noticed something wrong after hundreds conversions.
I have had to convert again many ...
Now I know this bug. More, I have all my configurations defined in Auto menu and I will only use avidemux version when I find this functionality.
I want to appeal on behalf of others , please change this default value to 'As Input' .
If it will be changed , it can help me too if I will find somewhere (on youtube?) the video with correct ration instead of wrong if the creator have not known about this parameter.
Yes it would be better to have "as input".
But you don't need to reencode evreything, just remux to copy/copy/mkv with proper configuration (bottom left button).