Personally I will prefer to have BSPlayer opening the files in real dimension (reading SAR and PAR) but I am not sure the problem is in the way it interprets this information or the filters generate this issue.

If the file is 480x480 (SAR) or even stranger than this, 480x576, that's obvious that the pixel aspect ratio it's not 1. But, this is ancient history, I don't believe someone uses those days such things. From a 480x480 you can have a 4:3 640x480 file. Usually, that's the real file, but you can also have a 720x576 file.