I think @Hooverdan is right. I’ve read the other forum thread they’ve linked to and I too believe that the host strings you entered fail the same custom validator validate_nfs_host_str, which does not expect a subnet nor a list of IPs.
I checked the issue list on GitHub, but couldn’t find an issue related to this there.
It’s worth mentioning that the last reply on the other thread explains a workaround that might work for you too, @kusyka911.