I have installed the daily pending iso of Xubuntu 25.10 and all is going fairly well in the VM I have in LVM/QEMU.
I can not find the /etc/default/grub file which I always edit to ensure the grub menu shows at every boot.
Has this file been removed or just moved elsewhere in the filesystem as it is not in my install in the expected place.
OK. Problem over!
I found a file /usr/grub/default/grub, copied it into /etc/default/ and edited it as I normally do and all is working as I want.
Is this the new expected situation or was this just a glitch in my iso file from 29th May?
Just that a glitch, But I too have seen this, rarely though. 
Good to know, thanks.
I will soon try the most recent version new pending daily iso and check if everything is there as I expect.
A new iso of Xubuntu 25.10 downloaded yesterday June 7th 2025 installed as expected in KVM/QEMU last night and the /etc/default/grub file was present and correct.
The lack of that file in the iso from June 4th or 5th must have been a glitch as you suspected; annoying but not disastrous!