The neon plasma 5.15.1 update-grub ext4 partition does not detect the Neon that is on the btrfs partition. I'm not 100% sure, but I think it broke up with the upcoming 5.15 version.

1/2) How do You see and know that?

2/2) How do we see and know that?

1.command sudo update-grub
2.

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$ sudo update-grub
[судо] лозинка за корисника lega99:
Sourcing file `/etc/default/grub'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.18.20--041820-generic
Found initrd image: /boot/initrd.img-4.18.20--041820-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Windows 10 on /dev/sda1
Found Manjaro Linux (18.0.3) on /dev/sdc11
Found Kali GNU/Linux 2.0 (2.0) on /dev/sdc7
Found Kali GNU/Linux Rolling (kali-rolling) on /dev/sdd3
Found Calculate/Linux on /dev/sdd6
done


Missing Neon on btrfs partition and Manjaro on btrfs partition
3. I checked /etc/grub.d half of the files has an invalid first line #! bin/sh instead #!bin/sh in all linux distribution which I have

4.

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[dragan@Manjaro ~]$ sudo update-grub
[sudo] password for dragan:
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-4.20-x86_64
Found initrd image: /boot/amd-ucode.img /boot/initramfs-4.20-x86_64.img
Found initrd fallback image: /boot/initramfs-4.20-x86_64-fallback.img
Found linux image: /boot/vmlinuz-4.19-x86_64
Found initrd image: /boot/amd-ucode.img /boot/initramfs-4.19-x86_64.img
Found initrd fallback image: /boot/initramfs-4.19-x86_64-fallback.img
Found Windows 10 on /dev/sda1
Found KDE neon User Edition 5.15 (18.04) on /dev/sdb1
Found Manjaro Linux on /dev/sdc10
Found KDE neon User Edition 5.15 on /dev/sdc2
Found Kali GNU/Linux 2.0 (2.0) on /dev/sdc7
Found Kali GNU/Linux Rolling (kali-rolling) on /dev/sdd3
Found Calculate/Linux on /dev/sdd6
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
[dragan@Manjaro ~]$


Only this grub is ok, I do not know where is CATACH-22

Example GrUB-entry for "other (non KDE_neon) Linux on BTRFS"

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$ cat /boot/grub/grub.cfg | sed -nE '/^menu.*ebian/,/}/ p'
menuentry 'Debian GNU/Linux 9 (stretch) (on /dev/sdb2)' --class debian --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-360e59c1-4bdd-466d-9049-4cb7fbf91264' {
        insmod part_gpt
        insmod btrfs
        set root='hd0,gpt2'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd1,gpt2 --hint-efi=hd1,gpt2 --hint-baremetal=ahci1,gpt2 --hint='hd0,gpt2'  360e59c1-4bdd-466d-9049-4cb7fbf91264
        else
          search --no-floppy --fs-uuid --set=root 360e59c1-4bdd-466d-9049-4cb7fbf91264
        fi
        linux /boot/vmlinuz-4.9.0-8-amd64 root=UUID=360e59c1-4bdd-466d-9049-4cb7fbf91264 ro quiet
        initrd /boot/initrd.img-4.9.0-8-amd64
}


And on the GrUB-command-line:
You could check wether the "prefix"-variable
has the right value.

I had to change it some years/months ago to;

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$ cat /boot/grub/custom.cfg | grep prefix
# Wrong: prefix=(hd0,gpt4)/@/boot/grub/grub
set prefix=(hd0,gpt4)/@/boot/grub/


On the other hand, I'm a bit worried, too.


# These two version numbers should agree with each other:

$ btrfs --version
btrfs-progs v4.15.1

$ uname -r
4.18.0-15-generic

But they don't.

I do not have disk drives with gpt, no UEFI, it is excluded in bios when I bought a machine and no linux could be installed. In every grub.cfg I do not have the line insmod btrfs, there is only:
insmod part_msdos
insmod ext2
set root = 'hd (X) 2, msdos (x)'
Incredibly identical lines are also in Majaro grub.cfg and all of them are listed here.
Yesterday Calculate had an update, the identical situation of the grub does not see the linux from btrfs partition.
I think that I take and pick up all grub.cfg files except two that are valid. U calculate has a special / boot partition, now I have the / root / boot partition all the same as the / boot partition, it should be / root / boot empty if I remember it well because / boot was created as a separate partition

I've changed the kernels from 4.15 to 4.19 and will not see btrfs.
The day I spent it, I give up

Edit:
I think I've solved the mystery. Both of my linux distributions have created the partitions themselves in the installation process on an extended partition. When I look at examples on the Internet, fstab / and /home have the same UUID. This is not the case for me, I created it on the extension partition / and /home as a separate btrfs partition. It does everything ok yet, but other linux do not see the linux distribution on those partitions.