I got it: it happened because I used ‘w’ with pacman (only download).
Then not until you start to install the update do you get
“Some packages should be upgraded first…
… archlinux-keyring-20220424-1 manjaro-keyring-20220421-1”.
When using -Suw, the download finishes before the keyring is updated,
and then the old keyring is used to check the downloaded files,
and packages signed with a new (unknown) key cause that error.
Could this be regarded as a bug in pacman?
It should update its keys before downloading with the ‘w’ option…