Same here.
The option in the preferences / Advanced / Browse / My Tag / Add "My Tag" to the "Comments" is working, BUT once you choose a track and you decide to edit a Tag, let's say an additional Tag like "Vocal", it disappears again in the comments. It was perfectly working in Version 5.
I'm running Windows 10 Pro, Version 1909 and Recordbox Version 6.0.0 x64
I would recommend fixing this asap because this is one of the killer features!