why my grub and linux efi not visible, directly boots to windows

I am new to linux, tried reading other post cannot find similar situation, thank you for you help and patience in advance.

I've set up a dual-boot system with Windows 11 and Kubuntu, each installed on a separate disk.(as in image)

I installed the bootloader on the EFI system file of Linux, which has a size of 977 MB. Furthermore, I created partitions for swap, root, and opt.

Disk 1 is linux:

Initially, GRUB loaded without any issues. However, I encountered ACPI issues and made some adjustments to the GRUB settings. Specifically, I added the following lines:

GRUB_CMDLINE_LINUX_DEFAULT="quite splash processor.max_cstate=5 rcu_nocbs=0-11"
GRUB_CMDLINE_LINUX="noacpi nomodeset"

These modifications allowed the system to reboot successfully. However, after some recent updates, GRUB is not visible.

Today after automatic memory test, the laptop automatically booted into Windows. I checked the BIOS settings, but there was no sign of the Linux option, and GRUB did not appear either, although the partitions remained visible.

What can I do to make the grub and other option visible again.

It's possible that the recent updates have overwritten the GRUB bootloader configuration or the Windows update may have changed the boot order in the BIOS. Here are some steps you can try to restore GRUB and make it the default bootloader:

Boot into Kubuntu using a live USB Open a terminal and run the following command to install the boot-repair tool:

sudo add-apt-repository -y ppa:yannubuntu/boot-repair
sudo apt update
sudo apt install -y boot-repair

Launch the Boot-Repair tool by running the following command:

sudo boot-repair

Click on the "Recommended repair" option and let the tool do its job. It should reinstall GRUB and set it as the default bootloader. Once the repair is finished, reboot the system and check if GRUB is visible.

This is strange behaviour for an EFI installation.

Windows generally only puts itself to the front on fresh installations, or when explicitly asked to. It might have happened as part of a feature upgrade, but Microsoft are usually more careful than that.

If this is an EFI installation, this can be repaired either with the Setup Tool, from Windows, or from Linux (which you can reach by using the Boot Menu).

This article explains how to reach the Setup Tool from Windows, or how to adjust the boot order from there.

You can also (usually) press a button at startup to go into the Setup Tool or open the Boot Menu. Which button depends on your mainboard manufacturer, it is usually F8 for the Boot Menu, and either the Delete key or F2 for the Setup Tool, but I've also seen F12 and a few others used. If you have a boot logo shown as the first thing, pressing the Escape key will usually give you a screen that explains what keys work. All of this is nonstandard and requires timing, so I recommend the approach of going through Windows for simplicity.