Edit: why am i forced to upload a photo for a new post?
Which way are you posting? Mobile/web interface? Shouldn’t need to add an image to post…
That aside, regarding your main question:
I think it’s very confusing when a single post appears with different comments on different instances, and have no idea how this works.
A basic reason for the difference in which comments are appearing across different instances/sites is because of delays in networking (federation) between the sites due to a variety of reasons. One of the common ones with the fediverse tends to be the software itself, and sometimes differences in versions’ federation handling. In this case it’s probably because Lemmy World is still running an older version of Lemmy with clunkier federation at Lemmy World’s scale, which causes delays in activities on there updating elsewhere (particularly those hosted in Australia).
Edit:
See also Kichae’s comment for a more detailed explanation, covers how things operate under more ideal conditions.
There’s an option when banning a user to also remove their content, albeit unless it’s an admin action I don’t think it would affect their whole post history beyond the specific community.
I’m kind of getting the sense as I look into this that it may be related to how Voyager is rendering the thread, as I’m not able to observe what’s being described from the web interface. That’s another catch in all this, the other apps and interfaces have their own quirks in how they handle rendering things, which itself is typically related to how they work off the base software (Lemmy in this case).