An option to bring in inputs without distorting the aspect ratio would be very helpful.

Currently vMix inputs scale to fit to the project resolution, but also stretch the pixels for certain aspect ratios, even when "Source" is selected. I don't know why, but this especially affects 9:16 videos. There is no way to display the video with correct proportions unless you manually horizontally scale it with the X Zoom parameter.

What I would like to see is if I bring a 1080x1920 file into a 3840x2160 project, the input occupies exactly 1080x1920 pixels on the output canvas, at zero Zoom.

Looks like 24.0.0.72 also now allows some 9:16 content come in correctly as long as "Source" is selected. "Normal" appears to be 4:3, and some 9:16 content still comes in this way whether it's set to Source or Normal. I'm not sure what causes the difference. Photoshop exported stills seem to come in correctly, and some MP4s, ProRes444 overlays are stretched to 4:3.

Learning to use UV maps, I was hoping to use these as a fix, but apparently they are applied after the input transformation.

Good idea.

Mediainfo shows what I expected, phone video is 1920x1080 so it must have a rotation flag that vMix ignores. I believe it used to obey it and stretch the video to 4:3 like it does with some other vertical content.

The bigger mystery to me is why some 1080x1920 content comes into vMix correctly at 9:16 and some gets stretched to 4:3. Mediainfo shows no difference. As far as I know the only standard non-square pixel aspect ratios are the squeezed ones for SD video, horizontal stretching is the opposite of this.