Hi,
Please check the explanation in
Avoiding apt upgrade from Jetpack 6.1 to Jetpack 6.2 - #6 by DaneLLL
Since the system is in r36.4.0 and there is new r36.4.3. Running sudo apt update and sudo apt upgrade will install r36.4.3, including the kernel packages. The solution is to run the commands first:
$ sudo apt-mark hold nvidia-l4t-display-kernel nvidia-l4t-kernel nvidia-l4t-kernel-dtbs nvidia-l4t-kernel-headers nvidia-l4t-kernel-oot-headers nvidia-l4t-kernel-oot-modules
And then
$ sudo apt update
$ sudo apt upgrade
To let kernel stay in the self-built custom kernel.