Ignore a rd.znet argument without subchannels. When using net.ifnames (the default), subchannels are used to build the interface name, which is required to match the right connection.
With net.ifnames=0 the interface name is build using a prefix and a global counter and therefore in theory it is possible to omit subchannels. However, without subchannels there won't be a udev rule that renames the interface and so it can't work.