A somewhat related report: I upgraded from stretch to buster (having started at wheezy, and gone via jessie) and my system also wouldn't boot with Xen 4.11 but was ok with Xen 4.8.
In my case the message was something like "error: unsupported tag: 0xc2c2". immediately as grub tried to launch it. In the end the hint in this report to manually run "grub-install" also fixed my problem. So maybe the root cause is indeed that it isn't happening automatically. No idea how dangerous it would be to do that though.
