Can confirm upon reboots I needs to do
sudo pacman-key --init
sudo pacman-key --populate archlinux manjaro
(more for the second one depending on your third party repos - should be anything you have installed as *-keyring at least)
And just that - then new installs work as expected (ex: sudo pacman -Syu cowsay).
I havent had the time to look at diffs or whatever to figure out why this is needed now though.