I heard a rumor that Pearl Mackie (Bill Potts) isn't coming back volgende season. What happened?

I'm pretty sure it's confirmed door the actress.

Her story has kind of been told, already? I mean, she had a sad, bittersweet ending, but I think it would kind of ruin the impact of Moffat's ending for her if she randomly abandoned her new adventuring companion to come back and hang out with the Doctor like old times. Especially if it happened so quickly.