Avidemux messing with aspect ratio

[Everend]

For some time, I have been recording off PBS broadcast TV using mythtv. I use Avidemux to cut off the beginning and end of the video (commercials for other shows) so all that's left is the show I want to keep. I've been using default settings for everything. The video and Audio output both set to 'Copy' so it only takes about 20 seconds or so to create the new file in mkv Muxer output format.
The original file is 704x480 and the output file is 704x480. The problem is when I play the output file using VLC or KODI the aspect ratio is messed up. I checked the setting on VLC it says it's using the Default Aspect Ratio. When I change it to 16:9 then it looks more like the original file.
So somewhere the default aspect ratio is being set different than the actual file height/width. Any idea where the setting is in Avidemux so it stops changing it?

thanks
Everend

[Everend]

Oh this is a recent change. videos I removed the commercials from a few months ago don't have this problem. I was using Avidemux 4.6.9 but last night I upgraded to 4.6.14 this new version is behaving the same.