BTW, and for my reference, this is mentioned in the MPF tag name documentation: https://exiftool.org/TagNames/MPF.html#MPImage although I don't expect anyone to find this. In other cases like this ExifTool issues a minor warning to inform you that the -ee option may provide more information, but I haven't done this for MPF.
Update to 13.61
Add the -ee option to extract information from additional embedded images.
Update to 13.60
OK. I have been unable to generate sample files or reproduce the problem with the MP4. Specifically, I need more details about the uuid box that comes after mdat in the original sample. An excerpt from a -v3 dump would help, but a sample would be better.
It may help if you could upload the files for a dual-file live photo for me to test.
I was able to reproduce this issue using a single-photo Samsung motion-photo HEIC file. The problem here was that the Samsung 'sefd' box must after both 'mdat' and 'mpvd'. This will be fixed in the next version, and affected files will be repaired if re-edited. However, to solve the dual-file JPEG+MP4 issue I will need to be able to reproduce this. I don't know how to produce a dual-file live photo with my Samsung phone.
This problem has been reported but will be difficult to track down. I hope it isn't an order dependency because these are really annoying to work around, but I haven't had time to look into this problem yet. It will be top of my list when i get a chance. Phil