I'm pretty sure I know what causes the bug, and it's that this https://github.com/detailyang/go-fallocate/blob/master/fallocate_linux.go library that butler uses since the last revision doesn't implement the behaviour of posix_fallocate from libc.
Your use of NTFS is another case in point, fallocate probably fails. Either Amos fixes this in butler/ox, or we make a feature request in go-fallocate, or the linux NTFS drivers implement fallocate. I'll file a bug report in ox and go-fallocate now.