I reproduced it on my machine (aarch64-darwin)

       > Did not find version 2021.01 in the output of the command /nix/store/vamzzqsav4nnyakgcq15v4z5gpqnvmw4-openscad-2021.01/bin/openscad --version
       >
       > Did not find version 2021.01 in the output of the command /nix/store/vamzzqsav4nnyakgcq15v4z5gpqnvmw4-openscad-2021.01/bin/openscad --help
       >
       For full logs, run:
         nix log /nix/store/qdyhkilahqf4ks8h0799f3jwyjagjdnb-openscad-2021.01.drv

for whatever reason it seems openscad --version is writing to stderr instead of stdout. I don't know the internals of versionCheckHook, but I suspect it wants the output in stdout

❯ /nix/store/vamzzqsav4nnyakgcq15v4z5gpqnvmw4-openscad-2021.01/bin/openscad --version > stdout 2> stderr
❯ cat stdout
❯ cat stderr
OpenSCAD version 2021.01

I observed the same with ...openscad --help, although this flag does not output the version

i get same behavior on my machine, weird.

       > Fetching https://github.com/kylef/PathKit.git
       > Fetching https://github.com/kylef/Spectre.git
       > Fetching https://github.com/onevcat/Rainbow.git
       > Fetching https://github.com/mxcl/Version
       > error: Failed to clone repository https://github.com/onevcat/Rainbow.git:
       >     Cloning into bare repository '/nix/var/nix/builds/nix-13475-1657500268/source/.build/repositories/Rainbow-64aa5363'...
       >     fatal: unable to access 'https://github.com/onevcat/Rainbow.git/': SSL certificate OpenSSL verify result: unable to get local issuer certificate (20)
       For full logs, run:
         nix log /nix/store/bqpd1d51vvg0g5nbj6ddj52plx5m1zfn-xcodegen-2.45.3.drv

versionCheckHook is supposed to check both streams (note the 2>&1)

versionOutput="$(@envCommand@ \
    --chdir=/ \
    --argv0="$(basename "${command[0]}")" \
    "${envArgs[@]}" \
    "${command[@]}" 2>&1 \
    | sed -e 's|@storeDir@/[^/ ]*/|{{storeDir}}/|g' \
    || true)"