Before working with heavy video files, ensure Autosave is enabled under Windows > Settings/Preferences > Preferences > Files/Projects to prevent data loss if a codec causes a crash.
If your video won't load, it is likely a missing system-level codec rather than a Maya bug.
Use a tool like FFmpeg or Shutter Encoder to export your MP4 as a series of images.
If you use an MP4 on an image plane, it may drastically slow down your viewport. If it does, switch the image plane's display mode to None when you aren't actively using it as a reference. codec issues - Forums, Autodesk