It does happen in Wavelab 9 too. And it goes all the way back to at least Wavelab 4, so it’s been like this for 15 (?) years.
It’s not just the Performer field, it’s any of the CD Text fields.
If you have a mixture of empty and filled fields for any particular field (like Performer - including Album and Track performer), all empty occurrences of that field will get an added space character. (As it is in your case, where you have Album Performer filled, but Track Performers empty).
But if ALL occurrences of a field are empty (album and tracks), none of them will get an added space character.
It happens on DDP and CDR. I don’t know why, and I don’t know if any other programs do the same thing.
btw, what external DDP player did you check it in?