GRUB issue

[Qqmike]

I don't have 25.04. I have checked 22.04 and 24.04.
If you look in your /etc/default/grub file, by some strange chance, you do not have anything like the following, right? -->
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true

(That would cause you to re-boot into the previously booted OS.)

You should have something like, simply, GRUB_DEFAULT=<some number, like 0, or 15, etc.>

[cookiemuncher]

In BIOS, ok.
But then, also, disable the Fastboot in Windows OS, too? Maybe as an experiment?

Disabling Fastboot in Windows also makes no difference.

[Qqmike]

Oh boy, I can't recall exactly what this is, I never used it, but there is a setting in one of the GRUB configuration files that -- I believe -- directs the PC to boot into the last OS it booted into. So, if you booted into Windows now, the next time you re-boot, GRUB will take you again to that Windows OS. Something like that. It's well known (but not by me! haha) Another shot in the dark. Suggesting that *somehow* that setting gets flicked 'on'? Frankly, it sounds crazy. But brainstorming. Some people suggest that it is Windows that messes with the UEFI boot order, but that also sounds a bit nuts.

[Qqmike]

Secure boot is disabled. Fastboot is disabled. Both in CMOS.

In BIOS, ok.
But then, also, disable the Fastboot in Windows OS, too? Maybe as an experiment?

[cookiemuncher]

Did some testing. If I select Windows 11 from grub but not enter my login pin, but then immediately reboot, then I find the CMOS boot order has already been changed.
I have been using Kubuntu 24.10 until 25.05 was released and this problem did not occur with that version.
Secure boot is disabled. Fastboot is disabled. Both in CMOS.

[TinyTim]

"Fast Startup causes Windows to not really fully shutdown."
You can hold the shift key down when shutting down windows and it should disable fast startup if you want to test it out. I think it is temporary however.

[Qqmike]

I haven't used or dealt with Windows since about 2008.
Someone here will probably have a definitive answer for you.
Windows somehow is messing with your boot order.
I have heard that this can be caused by Windows Fast Startup.
Fast Startup causes Windows to not really fully shutdown.
People have reported that they disabled Fast Startup (somewhere in your Windows Control Panel, Power, maybe?)
You have already discovered how to use your UEFI boot order -- which is one solution to this issue.
And you discovered that re-booting works if you stay in Kubuntu. (Otherwise, sudo update-grub might be relevant to try.)

[cookiemuncher]

I did a clean install when 25.04 appeared. I dual boot Kubuntu and Windows 11.
Booting into Kubuntu works just fine once setup. My problem comes after booting into Windows 11 and then later rebooting causes grub to be bypassed and boots back into Windows 11.
I then have to set Kubuntu as primary login in the CMOS setup again and can then enter grub with Kubuntu as primary and Windows as secondary.
Booting into Kubuntu repeatedly maintains the grub menu and setup.