Why has my "seen" feature in facebook disappeared in only one conversation?

[xyz]

There is only one conversation which I had on the 24th and my last reply appeared to me as "seen" until this morning (28th). How come my last reply doesn't appear as seen anymore? Is there an explanation for this?

[Callum]

The other party in the conversation can mark something as 'unseen'. It's nothing to worry, maybe it's just a bug. Just message the person back if it annoys you that much.

[Daniel]

the Person Who you Sent the Message to Probably Re Marked the Message as Unread If the Message Gets Marked as Unread than the Seen As Goes Away

[Drizzy]

They might have an extension that allows the user to automatically leaves the messages as Unread. Even if they open it, or maybe they didn't want you to think they're ignoring them, so they mark it as unread