FitzTheRuke
Legend
I don't see any rules that support the idea. It would be up to DM's discretion. This rule set basically leaves a lot up for DM interpretation.
That said, they don't need to see the attack coming if they're already wigging out from a magical darkness popping up and hear more spellcasting going on. If anything, they're probably moving around randomly just in confusion.
They are absolutely doing that! From a purely tactical standpoint, all darkness really does is make monsters move out of the area, but I like to roleplay my monsters and make them take a little while to figure that out.
As far as the game rules go, I think they mean for the save mechanic to work slightly differently than the attack mechanic (more than just reversing the who-roll-onus). I think they mean it to avoid the advantage/disadvantage mechanic as often as possible. I've never noticed this particular peculiarity before (which is strange in itself, because I've probably played nearly a thousand sessions of 5e - but then, I've never looked something up during the game before, I'd normally just make a ruling (probably in agreement with the player) and then move along and forget about it.)
In this case, I got curious and looked it up.