To be able to keep processes running which were started from a service unit after the service is stopped, you need to configure the following option in the .service file under the [Service] section:

KillMode=process

This keeps started processes from the service unit running after the service is stopped. This is not a recommended behavior, but my use cases involves a timer which calls a python script to run processes if needed, which needs to be running after the script has terminated. In the default behavior systemd kills all child process started from the service unit.

Here is the documentation: https://www.freedesktop.org/software/systemd/man/latest/systemd.kill.html