I don’t think you can “update” a file without re-opening it, it can be complicated to predict what happens exactly:
https://stackoverflow.com/questions/2028874/what-happens-to-an-open-file-handle-on-linux-if-the-pointed-file-gets-moved-or-d
but in general, don’t rely on the behavior, if you know you’re only appending, make Julia remember the last line number, and in the loop, re-open the file (and remember to close it) and jump to the line number.