Debugging these jobs is much easier with a gui like https://www.soma-zone.com/LaunchControl/
Here is another thread with tips on debugging agents and daemons.
In a nutshell, add keys to redirect STDOUT and STDERR to files you can tail.
To prevent jack to start at login or boot remove the file homebrew.mxcl.jack.plist from ~/Library/LaunchAgents or /Library/LaunchDaemons(depending on if you install it with sudo or not).
According to man brew:
brew services [subcommand]
Manage background services with macOS launchctl(1) daemon manager.
If sudo is passed, operate on /Library/LaunchDaemons (...