I noticed that his happens on our support project:

Note: Reporters are different people, just hid their names out of privacy. This usually happens when a lot of people reply to the same email. Almost as if the email shot out from Jira does not keep the issue key in the subject.
Is there a way to avoid this? I tested on my end, and when I open an issue, and reply to it, all the emails stay in the issue.
I am not sure if our customers are not really following this, or what is going on really (and I don't think I can ask our customers to check what's going on). So my question to you is: Is anyone experiencing the same behaviour? If so, how did you solve it?
Thanks a lot in advance!