sorry - no offense 
just changed to these lines - and it is still with a submenu
GRUB_DEFAULT="0"
#GRUB_SAVE_DEFAULT="true"
what happens if you just remove the submenu line?
Submenu screenshot
EDIT - just removed the lts kernel - grub still displays a submenu.
In my do-something-else-day I created the grub theme - so I have had some strange issues where the grub menu wasn’t displaying correct despite everything was as it should.
Not the background - only the menu - the background is courtesy of @anon96036739 here