Yes, there are actually three different ways of properly storing multiple values in MP4.
- Flatten multiple values
This is the most compatible approach, albeit not really multi-value. Here, multiple values are written in one field, separated by a character or character sequence, e.g.,;,,, or/
aART...data...Album Artist 1 Name;Album Artist 2 Name
- Write one MP4 atom per value
This is the approach Mp3tag and others have taken. Each value is stored in one MP4 atom, e.g.,aARTforALBUMARTISTwith adatapart that keeps the actual name of the album artist.
aART...data...Album Artist 1 Name
aART...data...Album Artist 2 Name
- Write one MP4 atom with multiple data parts
This is the approach Navidrome is using. Individual values are stored in multipledataatoms that are contained in the MP4 atom for the actual field.
aART...data...Album Artist 1 Name...data...Album Artist 2 Name
All this happened, because there is no clear standard on how multiple values are stored. iTunes / Apple Music doesn't really support multiple values as in 2. and 3., so different implementations came up with different ways of dealing with multiple values.
Mp3tag supports reading all three different variants (which I'd recommend for any application that reads metadata from MP4) and currently supports writing variants 1. and 2.
Since variants 2. and 3. are mutually exclusive, a dedicated configuration setting would be needed.
I'll keep you posted.