I'm using the quick selection tool to select my item then using the erase to remove the background but after the erase has been used the whole picture moves so that the picture can be fine edited. I've attached a picture which might explain it better.
Is something not working like it should? Let us know.
To be honest with you, that looks like a bug — could you email us about this at email@example.com with more details and possibly share the Pixelmator Pro document you're editing?