You probably missed this message during the update process
warning: /etc/default/grub installed as /etc/default/grub.pacnew
Changes to /etc/default/grub would not overwrite your configuration for GRUB because that could cause worse boot problems than the loss of one OS
checking the .pacnew file against the grub.pacnew (pacdiff or meld) would show this change for user to merge with their GRUB
# Uncomment this option to enable os-prober execution in the grub-mkconfig command
#GRUB_DISABLE_OS_PROBER=false