I know a similar question has been asked before but I haven't found a solution for MP3 files:
This has been a big problem for ages - text I enter in the comments field for MP3 files in iTunes is often missing in the Windows folder where my iTunes music is stored.
Sometimes, it also vanishes completely or appears as garbled code (just a series of numbers and letters, each 8 digits long) when the file is backed up.
for example, I just typed "balloons are wonderful toys" into the comments field of 10 MP3 files within iTunes - then I copied and pasted those files from within iTunes to a Windows folder. One file reproduced the text accurately - the other 9 all ended up with 9 different codes, one example: 00000405 000005C2 00001675 000029F8 00024AF0 00024AF0 0000637C 000071E7 0001B75D 000016C4
This is strange because I have a program that backs up my entire iTunes library (not individual tracks though) - and it always reproduces all the metadata every time - so I suspect this is an iTunes or Windows problem.
I'd really appreciate an answer to how to get around this so I can rest easy that all the metadata/tags are also backed up. The text in the comments field is very important for me.