Comment 3 for bug 1852581

I'm also getting the same issue on an ASUS VivoBook K451LB (x86_64, UEFI boot). 5.3.0-19.20 is the last kernel that boots on this system, all subsequent ones fail and drop to the initramfs prompt.

Unpacking /boot/initrd.img-5.3.0-19-generic and /boot/initrd.img-5.3.0-23-generic (using unmkinitramfs) and performing a diff doesn't appear to yield much, with only one file missing (an RTC driver that was intentionally removed).

However, I did notice that the kernel modules were just slightly smaller in the 5.3.0-23 initrd, and that eventually led me to this:

$ modinfo '/run/shm/initrd.img-5.3.0-19-generic/main/lib/modules/5.3.0-19-generic/kernel/drivers/ata/libahci.ko'
filename: /run/shm/initrd.img-5.3.0-19-generic/main/lib/modules/5.3.0-19-generic/kernel/drivers/ata/libahci.ko
license: GPL
description: Common AHCI SATA low-level routines
author: Jeff Garzik
srcversion: 20FB7D717055C5AA0AF9896
depends:
retpoline: Y
intree: Y
name: libahci
vermagic: 5.3.0-19-generic SMP mod_unload
signat: PKCS#7
signer:
sig_key:
sig_hashalgo: md4
parm: skip_host_reset:skip global host reset (0=don't skip, 1=skip) (int)
parm: ignore_sss:Ignore staggered spinup flag (0=don't ignore, 1=ignore) (int)
parm: ahci_em_messages:AHCI Enclosure Management Message control (0 = off, 1 = on) (bool)
parm: devslp_idle_timeout:device sleep idle timeout (int)

$ modinfo '/run/shm/initrd.img-5.3.0-23-generic/main/lib/modules/5.3.0-23-generic/kernel/drivers/ata/libahci.ko'
filename: /run/shm/initrd.img-5.3.0-23-generic/main/lib/modules/5.3.0-23-generic/kernel/drivers/ata/libahci.ko
license: GPL
description: Common AHCI SATA low-level routines
author: Jeff Garzik
srcversion: EF173B0C134561F058A30A9
depends:
retpoline: Y
intree: Y
name: libahci
vermagic: 5.3.0-23-generic SMP mod_unload
parm: skip_host_reset:skip global host reset (0=don't skip, 1=skip) (int)
parm: ignore_sss:Ignore staggered spinup flag (0=don't ignore, 1=ignore) (int)
parm: ahci_em_messages:AHCI Enclosure Management Message control (0 = off, 1 = on) (bool)
parm: devslp_idle_timeout:device sleep idle timeout (int)

Notice that the output of modinfo for the 5.3.0-19 module contains the following lines (which are missing in the 5.3.0-23 modinfo output):
signat: PKCS#7
signer:
sig_key:
sig_hashalgo: md4

My hypothesis is that the newer kernel modules being unsigned is the root cause behind these boot failures, since I currently have Secure Boot enabled. I'll attempt a boot with Secure Boot turned off and report back again.