Can't write tags to .m4b files...

Is there something I'm missing in 'Options' that needs to be checked or unchecked so this can happen? When I try to add a genre to an empty genre field, or add something to 'Album artist', I press the Save' icon, the 'saved tag dialogue box pops up, but here is no change to the empty genre field! No worries with other file formats, been using the programme for years, first time I've been stumped! Can anyone offer any help?

Audio books with chapter information are not supported by MP3tag.

Any chance that it may in a future version?

I can hardly say anything about the future. I don't even know who's gonna phone me in the next 5 minutes.
Here is a (German) thread from 2010 that describes trouble with M4b files:
/t/10644/1
So it has been 6 years....