Normally when using a Nostr client directly, when you reply, your message contains two or more special tags.

One tag is the original message id, and the other is the original poster id. That way Nostr apps can query to show you the previous messages in a thread, and also notify the original poster you have replied.

It seems some Japanese (and other posts for sure), are being copied from other networks, but without the linking data - so you cannot load the thread views and it looks like people are talking to themselves or just in global chat without using reply.