Reinstalling by itself does not suffice. Here is the problem :
For some reason, mkinitpcio now creates zstd compressed images instead of the previous default gzip.
kernels 5.4 and 4.19 cannot read these images.
A workaround is to uncomment
COMPRESSION=“gzip”
in /etc/mkinitpcio.conf, and rebuild with
mkinitcpio -P
I am not sure if there is a way to make 5.4 and below able to read zstd.