I read this about the m4p file format
A file with the M4P file extension is an iTunes Audio file, or sometimes called an iTunes Music Store Audio file. It’s really just an AAC file that’s copy-protected using a proprietary [DRM] technology created by Apple.
Perhaps this will affect importation.
I tried several mp3 and m4a files and had no problem importing them into Shotcut. They were not copy-protected files.
Can you share that mp3 file (or the features) to see what’s going on?