This likely has to do with how your home tab feed is currently set. If it says "All" at the top, it's showing posts from every single instance added in the app including default instances (if you haven't deleted them: beehaw, lemmy.world, & lemmy.ml).Thus the post that you're seeing, even if it exists on your instance as well, might not be the post from your instance, if that makes sense. I definitely found this behavior confusing myself at first and the devs are going to be talking about how this can be improved for new users. If you only use one instance and only ever want to see posts from one instance, feel free to long press your unused instances in the account setup page and remove them.

@zachatrocity we really have to stop having multiple instances by default.