This is really worrying and horrible. I edit for this publication and the very most I ever do, is add a separator at the end of a story and put the publication link at bottom. I would never in a million years ever alter a story like that. I only check for obvious typos and never change the writers style or how they choose to express themselves. This makes me feel very distrustful if our stories aren't even safe on Medium. No one should ever tamper with another writer's story! And plagiarising your content is 100% unacceptable and very worrying. I would be freaked out if this happened too.