So… Is there a way to check when a kernel got swapped on my desktop/PC? I don’t know when this happened but I’m suddenly running 6.18.1 today and I haven’t done the usual manual swap with the grub bootloader to the latest LTS kernel.
I did install the stable update 2025-12-15 this morning, and installed the kernel as soon as i saw it in stable update 2025-12-08
So there’s your answer. 
You installed 6.18 yourself, and considering that 6.18 is the latest kernel from upstream, it has by definition a higher version number than whatever kernel you were running beforehand.
As such, if you do not manually select a specific kernel in the grub menu — under the “Advanced options for Manjaro Linux” submenu — then grub will default to the kernel with the highest version number. 
Okey, sooooo. Even though I hade previously manually selected the kernel 6.12 way back in February, was diagnosing if a thing was a potential issue for myself, that choice didn’t matter in this case?
I was jumping between 6.6 and 6.12 just to clarify
No, it didn’t. Now that Manjaro defaults to btrfs, grub can no longer remember the last-booted entry anyway, because in order to do that, it has to be able to write to the filesystem that /boot is on before the kernel gets loaded, and grub cannot write to btrfs — in some cases it even has problems reading from it.