Quoting the bz where it was first reported:

tripleo_kernel_args is not applied when the new KernelArgs is subset of preceding one.

Let me assume that the following situation.

1st deployment with KernelArgs: "isolcpus=1,3,5,7,9,11,13,15"
2nd deployment with KernelArgs: "isolcpus=1,3,5,7,9"

At the 1st command, KernelArgs was successfully applied.
At the 2nd deployment, the new KernelArgs option is not reflected to /etc/default/grub.

Since the edpm_kernel role was inherited from tripleo_kernel role, we need to fix this bug for 18 as well.