It seems we are missing some shell-launching fallback case when ENV has been cleared.

r, w = IO.pipe
ENV.delete("PATH")
success = system("echo", "hello", out: w)
w.close
p success
p r.read

On CRuby, this runs successfully, returning true for success and the expected output is read.

On JRuby, the system call fails, returning nil. Removing the ENV modification fixes it. @mullermp confirmed for his example that deleting PATH from ENV is enough to break it, so I suspect we are failing to find the shell with PATH and not falling back on /bin/sh or something like that.