It appears multiple instances of /editbox defaults into only the last one being sent.

example:
//editbox -an A | editbox -an B | editbox -an C

even in debug, none of the other lines were sent, except for the last one, "C"

Notes:
mIRC 7.66 on Windows 10

See here for previous discussion.

You can try adding timers on every next line to the command:

Khaled: A work-around to the SendMessage/PostMessage limitation would be possible if we had the means to use the /editbox command to populate multiple lines into multi-line editboxes. Then the batch could be sent all at once.



As of right now, $crlf nor $lf nor $cr can be used to stimulate multiple lines in the editbox, and it makes me very sad and gosh dang frustrated. :'(

Yes, I also this noticed  :-[  and it would be very useful to improve (develop) this function.
In addition, it would be helpful if we got a command to resize of the "Editbox" field to modes: "/editbox -m [Single,Multiple,Automatic,Pixels [N]] [window]".
And also, so that this field has the ability to automatically resize (in "Automatic" mode) when used the identifier $crlf for line break (linefeed down), when there is only one line in the text.

All this would be useful when creating scripts associated with this field for editing text.