If I’ve done everything correctly, the 21.05 ISO also passes the check on my machine, hoorray!
Feel free to verify everything yourself; getting @grahamc’s r13y script running is trivial (I’d recommend running it inside a Docker container, though remember to enable sandboxing!)
The paths are “unchecked”, not “unreproducible” - which means they might in fact be reproducible, but https://www.r13y.com was unable to verify this. This could have any number of reasons, including things like timeouts or exhausted memory or disk space.
For example, manually checking nix-build /nix/store/310735bj7rnfhy1vyrddalz2cfa9f4kl-gzip-1.10.tar.xz.drv --check suggests that derivation is actually fine AFAICS.