preseed/late_command is interpreted differently depending on how the file is passed to the installer

If I give the installer a preseed file using url=tftp://10.30.112.109/ubuntu/preseed.cfg on the kernel command line, the following late_command works fine:

d-i preseed/late_command string tftp -g -l /target/root/bootstrap.sh -r /ubuntu/bootstrap.sh 10.30.112.109 && mv /target/etc/rc.local /target/etc/rc.local.orig && printf '#! /bin/bash\nbash /root/bootstrap.sh 2>&1 | tee /root/bootstrap.log\n' > /target/etc/rc.local && chmod +x /target/etc/rc.local

However, if I rebuild the initrd and put the exact same file as /preseed.cfg in the file system, the command is cut off after the first occurrence of \n, so it tries to execute

  tftp -g -l /target/root/bootstrap.sh -r /ubuntu/bootstrap.sh 10.30.112.109 && mv /target/etc/rc.local /target/etc/rc.local.orig && printf '#! /bin/bash

I tried double-escaping the newlines as \\n, but this just appended a \ to the end of the command line.

I would expect the same preseed.cfg to be parsed the same way, regardless of method of delivery.