Always report plagiarism. The platform will flag it and will take it down if they determine it to be plagiarism. Plagiarism is illegal.

There are a couple of easy things you can do. I, for example, cross post on other platforms simultaneously. My creative writes are always published on a creative writing platform which gives every new post a copyright stamp. I have multiple copies of my own creative work under my own name in different place – all with publishing date stamps. It's not a perfect solution, but it can help establish your authorship.