Yes, I used Windows explorer to tag my music mostly. I am using MusicBee for only a month. Thats why it would be hard to edit all the files in MusicBee now.
I have no idea why MusicBee does that, but I can offer a solution:
Select all the tracks with multi-value tags that MusicBee has now separted by a forward slash /
Then go to Tools > Tagging Tools > Search and Replace...
Search for "/" (without the quotes)
Select the <artist> field
Replace with "; "
By doing that, MusicBee will update the values accordingly.
Then going forward, to make ur life easier, just do your tag editing using MusicBee itself.
I already spend hours on end on social media. Might as well spare a few of those to a greater purpose here.