The way that bootc handles kernel arguments is markedly different from the way that rpm-ostree handles them. While rpm-ostree provides CLI flags and arguments, bootc looks for files in /usr/lib/bootc/kargs.d. Given that this directory is in the immutable part of the OS image, this means that an image build must occur whenever a kernel argument changes. 

For more info: https://bootc-dev.github.io/bootc/building/kernel-arguments.html