Grub2 updates for Red Hat systems are making some unbootable

I generally stay as far as possible from UEFI and where I have a choice of bootloader I like syslinux. Grub seemed like (was) a great advance on LILO in many ways but Grub2 is a huge confusing mess.

I wish I'd read this article before updating practically all my grub booting servers (CentOS 7 and Debian 10)! Here's hoping there's a fix that can be applied before any reboots are required...