EDIT: Possibly one could say that this is not a bug, then I would still feel it as annoying or incorrectly displayed because this person had not made a direct mention but was only replied to a message. This problem could well lead to confusion.
Had the same thing today. @LeonMarcelHD said it could be that this happens when the original message is not loaded in the chat history. Could really be just that since it indeed was not loaded in the history. By scrolling up I just loaded the past 2 weeks messages.