Hypothesis: in the script you did set -o pipefail (if the shell interpreting the script is Bash; other shells may provide similar functionality). From man 1 bash:
pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.
When there is a match, grep -q exits early and returns 0. Then echo may or may not return 141. Even for the same input things may go one way or another, it's a race condition. If 141 happens then the return value of the pipe will be 141 because of pipefail.
My answer under the linked question provides solutions. The simplest one for you is:
if (echo "$pid_list"; true) | grep -qF "($script_pid)"; then