in my project, 640 x 480 webcam footage is intercut with 720p footage, as well as with still images that have been previously cropped at 1280 x 720 in order to match the 720p footage. The first clip on the timeline is one of the low-res webcam clips. However, In Project > Project settings, the profile HD 720p 25 fps is explicitly selected. Even though a 720p profile is selected, the output is 640 x 480.

I've also played around with the rendering configuration, using the MLT Framework codes. I tried setting display_aspect_num and display_aspect_den, as well as width and height. Although that definitely affected the output, I gave up on trying to learn the MLT Framework sufficiently, and instead began editing each of the webcam clips in Shotcut in order to change their aspect ration to 16:9 before importing into Kdenlive.

I would not have thought of this as something worth reporting, but after discussing this in a couple places on Reddit, I decide to look into the possibility of this behavior being a bug. I'm wondering if this might be a bug, and if I should report it somewhere.

Rescale switched on?