It's definitely a feature (http://man7.org/linux/man-pages/man2/open.2.html) and it's controlled by underlying OS, not golang runtime.
O_APPEND
The file is opened in append mode. Before each write(2), the
file offset is positioned at the end of the file, as if with
lseek(2). O_APPEND may lead to corrupted files on NFS
filesystems if more than one process appends data to a file at
once. This is because NFS does not support appending to a
file, so the client kernel has to simulate it, which can't be
done without a race condition.