I have more information. I am using this version of restic on my Windows 10 machine:
restic 0.9.3 (v0.9.3-62-gc8fc7236) compiled with go1.10.2 on windows/amd64
I do most of my work in the Cygwin command window, which is a Unix-like wrapper.
If I run the command in the stock Windows Command Prompt window, it works fine. If I run it in the Cygwin window (mintty) it is truncated.
The window is properly sized before I run the command - I am not resizing it while the restic command is running.
So it looks like Cygwin may be messing with
restic's ability to determine the correct window size.