linux - sed needs `\n` to append new line

I am trying to append a new line using sed, it works only when I add the new line like this: \\\n:

echo "sometextutf8_filesystemmmmm" | sed -r "/utf8_filesystem/a \\\n# Passive mode"

the output:

sometextutf8_filesystemmmmm

# Passive mode

One or two back slashes do not work! With one \n or two \\n back slashes I just get this output:

sometextutf8_filesystemmmmm
n# Passive mode

without any new line.

Even though, it works properly without having three backslashes with substitution:

echo "sometextutf8_filesystemmmmm" | sed -r "s/utf8_filesystem/\n# Passive mode/"

Output:

sometext
# Passive modemmmm

Could some one explain that behavior?