This indicates that Grub is already having problems decompressing the kernel. Possibly not just with the initramdisk.
That would mean something went wrong when writing the kernel. It’s already precompressed, and compression is attempted again when saving to Btrfs.
There seems to be an incompatibility between the saving of the compressed kernel xxx by the kernel yyy and the btrfs file system driver zzz.
To trigger this, mkinitcpio cannot be used; instead, the kernel in question must be uninstalled and reinstalled.
It might also be helpful to compare the kernel checksums after they have been written. But of course, that won’t help if the error lies with Grub.
This is possible using maxi
P.S.:
When i installed the update to 6.15, i had kernel 6.12 running