is the filesystem mounted read-write, or read-only ?
Yes, it is. I was sure I had it mounted read-only, but I don't. I will fix that. There's no sense in mounting a filesystem read-write over a read-only device...
In the meantime, I set the samba share to read-only and this prevents the problem. I will mount the filesystem read-only and test again when I get a chance.
So this seems like a "don't do that" problem although it would be nice to fix it. If it happened with one of the userspace filesystem things I suppose that wouldn't be so good. I haven't tested any of that.