Alt+S works differently than Ctrl+S would
As specified here: https://wiki.archlinux.org/index.php/Readline#History
using Alt+S aka:
doesn't work the same as it would work if using Ctrl+S or a manually set Ctrl+Y for forward-search-history
the Alt+S variant clears the searched string when pressed (I think it's because the Alt key is held), so it cannot be used to successively go forward, like Ctrl+R is used to go backward to the previous one successively every time it's pressed, once the search string is present. Ie. try Ctrl+R type grub (or anything else that you might have in history, more than one time) then Ctrl+R again it will find the previous one. But, Alt+S type grub then Alt+S again, it clears the searched string(which is grub in this case) and cannot be used to continue the search forward.
I use Ctrl+Y in /etc/inputrc aka: