Hi, welcome to the forum!
I don't see a bug here, just incorrect tags. MusicBee definitely doesn't add // to multiple tags. If you're sure that all the artists are correctly split (check with the [...] button to see multiple artists), then you probably just need to remove the display artist value, which you can do with the tag inspector or Remove Tags from the Tool menu.
Edit: After looking at your screenshots again, you definitely have some artist values that are not split. Anywhere you see the // divider in the artist browser, open the multiple artist splitter to make sure that the separate artists are there. Then you can remove the display artist and it will show correctly