The grub.cfg used by Ubuntu Core is here. I tested running update-grub which overwrote /boot/grub/grub.cfg. On reboot I caught the system in the UEFI Firmware Settings menu, pressed c for the command line and run
chainloader /EFI/ubuntu/kernel.efi snapd_recovery_mode=run
boot
This booted without splash screen, I had to enter the recovery key in the text console and system booted into the full desktop. I downloaded the grub.cfg:
curl -LO https://raw.githubusercontent.com/canonical/snapd/refs/heads/master/bootloader/assets/data/grub.cfg
moved it into the /boot/grub/grub.cfg and rebooted. Now it was with normal splash screen, I had to enter the recovery key again and the next reboot was normal.
If you customized the kernel command line it is written in the /boot/grub/grubenv and you can load it at boot time with load_env command (see the grub.cfg).
My understanding it that grub.cfg is written when snapd runs in installer mode, maybe when it is updated.