fediverse
Probably yes, depending on exactly the sort of case you're referring to.
If it helps: posts don't live on the origin instance. They are broadcast to other instances, and basically every reply is a brand new post, with a reference to the old post, that also gets broadcast to other instances.
Checking out the standard, though, it leaves it optional ("may") for clients to notify the origin instance that there was a reply.
At least, if I read it right.