Shell command does not execute and I'm losing my mind

I've been writing a simple shutdown utility in Rust to gracefully close all open apps before running a final command and exiting: https://github.com/dastarruer/gtkshutdown. This final command is provided by the user. However, this final command just... doesn't run after the program finishes. Or, rather, sometimes it does, and sometimes it doesn't? I'd really like to finish this project and move on with my life, but I just can't figure out what the issue might be. To test this behavior, you can...