Why exit code 141 with grep -q?

This is because grep -q exits immediately with a zero status as soon as a match is found. The zfs command is still writing to the pipe, but there is no reader (because grep has exited), so it is sent a SIGPIPE signal from the kernel and it exits with a status of 141.

Another common place where you see this behaviour is with head. e.g.

$ seq 1 10000 | head -1
1

$ echo ${PIPESTATUS[@]}
141 0

In this case, head read the first line and terminated which generated a SIGPIPE signal and seq exited with 141.

See “The Infamous SIGPIPE Signal” from The Linux Programmer’s Guide.