@TimePencil

If it happens again, as well as screenshotting it, might be worth checking if the same toot is in your home timeline when using both apps and your server's website.

If it's just on one app, then there may be a bug in that particular app which is somehow mixing up timelines.

If the same stray post is on multiple apps and the website, then there may be a bug in the software running on your server.