Maybe rebuilding the ramdisk failed during the original upgrade?
One of the post-install stages after upgrading the kernel is rebuilding the initramfs - a tiny environment for bootstrapping the main OS.
If you trigger it manually with mkinitcpio --allpresets you’ll notice it has fancy colorful output, with clearly visible warnings/errors. However when invoked as part of an upgrade this coloring is removed, making errors difficult to spot.
I had this stage randomly fail a few times, resulting in an unbootable system like you described - solution was to just trigger a manual rebuild or reinstall the kernel with pacman -U.
It’s possible that this is what actually fixed things when you downgraded the kernel.