So i compiled a Linux kernel, installed it, ran mkinitcpio (all via this guide here https://wiki.archlinux.org/title/Kernel/Traditional_compilation), ran grub-mkconfig -o /boot/grub/grub.cfg, it detected my new linux kernel, rebooted, and, no new kernel.
Does the entry appear in grub.cfg, is there a Grub menu entry you can select? Are there any error messages? Looking at some kernel PKGBUILDS in the AUR should give some general up to date guides. Older kernels might not support zstd mkinitcpio compression so use lz4 in /etc/mkinitcpio.conf and it's possibly better anyway.
I don't have this problem so you are missing something.
Did you highlight / press ENTER on
in the grub menu presented at boot time?