I don't doubt that. And I am pretty sure that MP3tag adds accurately the metadata to the files.
And I am also pretty sure that the metadata is properly displayed in MP3tag and it is also embedded in the files.
What is not so sure: how to other programs treat WAV files? Can they read the metadata? Can they read all the metadata?
E.g. the Windows Explorer reads wav tags only to the extent of former ID3V1 (which is not the same as the chunks in wav files) tags - and that does not include pictures and no ALBUMARTIST either.
So if you test the presence of metadata in wav files with other programs than MP3tag, you have to make sure that this program can cope with wav files to the same extent that MP3tag can.
I just googled a little and found a lot of threads that complain about Apple programs that cannot cope with wav files properly.
That is why I ask about the functions in Mac OS.
If you have sample files that behave properly, then please show us these or screenshots of the relevant information dialogues.