I have the same problem with btrfs on Manjaro as I had earlier with Arch: As long as I only want to boot the latest kernel, everything is fine. However, if I want to go back to the last LTS kernel and therefore set the value in the grub configuration to
GRUB_SAVEDEFAULT=true
in addition to
GRUB_DEFAULT=saved
to boot with the last selected kernel at every startup, there is first an error message or prompt, then after timeout it boots again with the current kernel. The reason, I found out at the time with Arch, is that Grub can not be processed on a vfat partition (boot/efi). The only remedy was to create a separate boot partition in ext4. The way the automatic installation is set up with btrfs, there is no boot partition. If I have to select the kernel manually every time I boot, it is worthless for me. How else can the problem be solved? I would like to switch to btrfs, was looking forward to Manjaro supporting it better now.
Now I discovered in the /etc/default/ folder, where grub is located, the folder grub-btrfs, which contains a config (currently ext4 is active again). Is it perhaps possible in this config to solve the problem and keep the last booted kernel? If so, which entry does that?