How to debug grub2-efi -> kernel transition issues?

[Richard W.M. Jones]

I have a weird problem on an old Asus Zenbook UX305C where new kernels
cannot be installed by grub.  Specifically what happens is they appear
in the boot menu fine, but if you try to boot them then the machine
hangs hard with a completely black screen.

Oddly the kernel installed by Anaconda can boot, but obviously this
makes upgrading the kernel RPM impossible.

My real question is how on earth do I start debugging this?

I edited the kernel command line, removed “rhgb quiet”, added
“loglevel=9 nomodeset” but still no output at all is visible before
the hang.  The machine doesn't have a serial port.

Any ideas?  Does grub have debugging that can be enabled somehow?

Rich.