Are you using Apex class as the InboundEmailHandler or the old email2case (where you had to install an agent within your network)?
If it's Apex - are you sure you save the addresses on the EmailMessage record?
Something like this:
EmailMessage newEmail = new EmailMessage(
FromAddress = email.fromAddress,
FromName = email.fromName,
ToAddress = (email.toAddresses == null || email.toAddresses.isEmpty() ? '' : String.join(email.toAddresses, ', ').abbreviate(4000)),
CcAddress = (email.ccAddresses == null || email.ccAddresses.isEmpty() ? '' : String.join(email.ccAddresses, ', ').abbreviate(4000)),
// skipping some stuff...
Subject = email.subject,
Incoming = true,
MessageDate = System.now(),
Status = '0',
ParentId = caseId);
Last but not least: I don't have an org handy where I'd have both email2case and Chatter but if you click the standard buttons there's a difference between "Reply" and "Reply to all" ;)