By default, Slurm is configured such that it allocates an entire node to a job which requests a subset of the resources.
You need to configure the partition to be shared.
In slurm.conf on the headnode, you'll need to add the following lines outside of the autogenerated section and restart the slurmctld service:
Please note, changing the SelectType plugin and restarting the slurmctld service may result in terminating all the running jobs.
You may also need to run the scontrol command on the headnode to inform the compute nodes of the change.