Problem parsing complex values in grub-config-flags

So I'm trying to properly isolate a real-time workload on OpenStack, and need to set "nohz_full" and "rcu_nocbs" for the same cores as was set in "isolcpus". There are no dedicated settings in the charm so I rely on setting grub-config-flags.

$ juju config sysconfig grub-config-flags='GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT nvme_core.multipath=0 nohz_full=22-39,62-79 rcu_nocbs=22-39,62-79"'

Renders (notice, a comma becomes a newline):

GRUB_CMDLINE_LINUX_DEFAULT=$GRUB_CMDLINE_LINUX_DEFAULT nvme_core.multipath=0 nohz_full=22-39
62-79 rcu_nocbs=22-39,62-79

while I would expect is to be:
GRUB_CMDLINE_LINUX_DEFAULT=$GRUB_CMDLINE_LINUX_DEFAULT nvme_core.multipath=0 nohz_full=22-39,62-79 rcu_nocbs=22-39,62-79