There has been a grub update recently, apt should have asked you if you want to install new /etc/default/grub file or keep current. Look if you have /etc/default/grub.dpkg-old and compare the contents.
I assume it could be that the line '#GRUB_DISABLE_OS_PROBER=false' got commented, such os-prober will not run to search for other OS on other partitions during update-grub.