Solved. I’m now getting the correct output for showmount -e localhost as root.
NFS Settings window below. I had disabled NFS v3, as I don’t have any clients that use it. This is something I was able to do without a problem on my QNAP, so it didn’t occur to me that it might be a problem, but after doing more research last night, I ran across an older thread where someone else had a different RPC error because they had disabled NFS v3.
I’m going to file this as a bug report because this doesn’t seem like correct behavior. There was nothing in the docs I read about it. I believe that either it shouldn’t break NFS v4, or it should throw a warning that it’s going to effectively disable the NFS shares.