I have a .mkv video file I need to use but it is apparently infinite
So I've found the issue to be related to inconsistent framerates. I found that running the video through ffmpeg with ffmpeg -i yourfile.mkv -c:v copy -c:a copy -r 30 newfile.mkv actually fixed the issue entirely. Now I can open the file correctly and it appears to be 42 minutes as expected. More information here.