Export Frame problem

I am trying to export a still shot from a mp4 video using the Export Frame option in the file menu.
The video’s aspect ratio is 16:9 and resolution 720 x 576. The still shot, be it a png, bmp or jpg file, appears
thinner than the original. By thinner I mean it is the same height but narrower and therefore not usable.
The resolution of the still image is reported as 720 x 576 and the pixel aspect ratio is 1:1.

Tux Designer in his YouTube video entitled “Pause Video or Hold/Freeze Frame in Shotcut” does exactly what
I am trying to do but his aspect ratio doesn’t change.

Any help would be appreciated as I am clearly misunderstanding how this works.

Thanks

Nigel