The exec node uses the node.js child.process module either in exec or spawn mode. Have a read at
https://nodejs.org/api/child_process.html
for a lot of info on how they work on different systems.
Unfortunately their is quite a few little caveats in how this module spawns processes that can lead to things like orphaned processes and so on.
Nodered has very little influence on how this happens as the exec and daemon nodes are pretty much simple straight forward wrappers around some of the functionality of the child.process module.
So probably your problem lies on the node.js side and not the exec node implementation side so you d have to do research there about this behavior.
Johannes
