I did some experiments to see if I could reproduce the track not showing up, but I haven't been able to.
It could be that having ID3v1 as well as 2.3 tags in the file is contributing to the confusion. What you see as MusicBee "adding" slashes may be a value from the v1 tags being imported to the 2.3 tags. I tried removing a particular tag from 2.3 in my tests, and it reappeared because it was still in the v1 tags.