Apologies for the delay on this one! It looks like your email was moved to our unsuccessful folder since you forwarded a ticket from your system with the reply token still intact.
In any case, when making a feed comment and notifying multiple people, replying to the original comment does not perform a "reply all" to all originally notified people, only the people on the reply chain. For example, User A makes a new comment and notifies Users B, C, and D. User B replies through email to A's comment, only A is notified. User C replies to A's comment, Users A and B are notified since they are both now on the reply chain.
We have a Knowledge Base article explaining this in further detail. You can find the article here:
Apologies for the confusion! Please let me know if you have any further questions or if there is anything else I can do to help!