I just downgraded initramfs-tools to v=0.130 and it all works again, also with new kernel 4.19.122-2-osmc. Just for comparison, I also have a RPi4 running raspbian buster on kernel 5.4.51-v7l+, and there are no issues there (same btrfs setup, same procedure for creating initrd). The raspbian initrd file also has things moved to the /usr directory, but it creates no issues there.
Just FYI although technically off-topic here, I may have managed to find a way to support btrfs in the native osmc initramfs, which would negate the need to have a separate initrd built with initrafs-tools.
However, IMHO the issue in this thread should still be fixed, as somewhere there is a break in the upgrade to Buster.