Long m4b file error

I have a long audiobook in m4b (AAC) format which is over 20 hours long. I have never had any issues tagging m4b files except for their obvious limitations. However this very long file loses all it's tags when it is opened in mp3tag. It won't save tags either. I have heard that there are various issues with very long m4b files but this is the first time I have had this issue. Does anyone have any ideas if I can get around this without splitting the file? Is this a known issue?
Regards