substituteStream warning during nixos-rebuild

Rebuilding configuration.nix gives many warnings of the form:

substituteStream(): WARNING: pattern <pat> doesn't match anything in file <pat>

where <pat> is one of /bin/mount, /sbin;/blkid, /sbin/mdadm, /sbin/modprobe, /usr/bin/basename, and /usr/bin/readlink and all of the <files> are from udev-rules. I believe it started happening after upgrading to 21.05 (or maybe 20.09). The warnings appear to be innocuous. Why does the warning exist and shouldn’t it removed?

They come from fixing the paths in udev rule files – not all files contain all of these paths so you get the warning: