Still, echo-message was designed by the IRCv3 team as a replacement for znc.in/self message, there is absolutely no point to integrate it for everyone if they don't even use a ZNC + buffer.
This is not a ZNC-specific feature. You can read about echo-message and its purpose
here.
I was not able to reproduce the issue you described in your original post. This is the reason why I was asking for a step by step method to reproduce your issue. If you do not provide a numbered, step by step method, that you have tested and confirmed reproduces your issue, then it will be impossible for me to know what the issue is or how to reproduce it.
Based on the debug you provided in your post, it looks like you are reporting the same issue that was originally reported with self-message? ie. that incoming messages with source as your nickname and target as another nickname (which is not a normal or expected combination for an incoming message) are being displayed in a different window?
