Inconsistent behavior when forwarding an external email to zendesk

Hi there, would like to raise a concern and improvement suggestion for the current ticketing system.

Due to work requirements, my organization have to first manually forward from an external mailbox to Zendesk, after which we will work on that external email and reply again via bcc to the same Zendesk address.

When I forward and/or reply bcc an external email twice to zendesk, I observed 2 separate behaviors despite doing the exact same steps.

1st behavior: it creates 2 separate tickets
2nd behavior: the 2nd time when I forward from the same email source, it goes back to the original zendesk ticket. (This is the ideal behavior).

It creates unnecessity as I have to manually check to see if it creates a separate ticket OR goes back to the same ticket thread, resulting in inefficiency in my work.

I have communicated with a Zendesk Support Agent and he/she had escalated the issue, and the outcome is zendesk is not able to understand why there is such inconsistency, and I was recommended by the Agent to make a post here. (On a side note the agent had provided prompt and great assistance despite not being able to solve the issue, not his/her fault though).

If further details are required, feel free to reach out to me in this account or to my personal email. 

Thank you for your attention.