From what I can gather, you do (or did) not have group messaging enabled. You were on the receiving end of a group message. Replies to that group message also came to you, a recipient of the initial group message. Because you do not have group messaging enabled, the messages which were in response to the group message were not grouped.
I know I sound pedantic, but I do QA. If we're calling it a bug, let's get some expected results and steps to reproduce.