In the question there are two sub questions to answer.
elementary now automatically boots up just fine without showing GRUB
This is normal behaviour. (since It is not dual boot )
If no other operating system is detected GRUB will boot straight into the default operating system and no menu will be displayed.
If another operating system is detected the GRUB menu will display.
Also elementary OS is configured on the system as the default, I think the 'wait timer' is so short that it'll default-boot into the latest kernel.
Menu will appear if you press and hold Shift during loading Grub.
To get GRUB menu permanentaly you need to customize the
change settings GRUB_TIMEOUT=10
and place a "#" symbol at the start of line GRUB_HIDDEN_TIMEOUT=0`
Save changes and run
sudo update-grub to apply changes
Will GRUB show up once there's a kernel update
No .Kernel update never trigger grub to show menu at boot time, unless you do something to
/etc/default/grub before and forgot to run
Also whenever kernels are updated, grub push the old kernels back to advanced options and keep new kernel as default main entry.