Kernel modules are not getting built after a change in linux

I am using openwrt v19.07.2 release and my custom linux version is 4.9.x
I have a kmod package which using a linux header file.

If the linux header file is modified, the kmod package should re compiled during "make -j42". But it is not happening.

If i do make kerne_menuconfig and save, things are working as expected.

Is this a bug? or any specific reason behond this behavior?

Steps to reproduce.

  1. Note down the timestamp of a siit.ko from staging_dir

  2. update linux kernel header file "include/linux/netdevice.h" since siit.c include this file.
    vi ./build_dir/target-mips_24kc_musl/linux-ath79_generic/linux-5.4.108/include/linux/netdevice.h

  3. re-compile openwrt and check the time stamp of siit.ko.

timestamp of siit.ko is not changed. But as the linux header has changed the kmod also should have been built.