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When installing a newer kernel-rt, the kernel-rt-firmware is *upgraded*. When booting the previous RT kernel, it might cause the boot to fail as the proper firmware is not available. The old kernel will most likely not know about the newer firmware, as it is placed under /lib/firmware/$(uname -r). The kernel-rt-firmware package should not be upgradable, but be able to be installed side-by-side, just like the kernel{,-rt} packages.
This has been fixed for a while. Closing.