Hello im here to report something so it'll get fixed. Im using the latest version of essentialsX b463 but i think that this also happens in older beta and releases.
So my todays issue is that. If i contact someone with /m and after this someone else contact me, when ill try to replay to the latest person that contacted me, i will not replay to him, but to the first guys. ( to the one that i used /m to contact him )
I guess you have understood the issue, thanks for the great work.

This is a config setting - last-message-reply-recipient. This is not a bug.