When granted advantage on an attack, are spells that cause saving throws rolled with disadvantage?

Question

The PHB states that for being granted advantage...

Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. When that happens, you roll a second d20 when you make the roll.

and for acid splash

Acid Splash... Choose one creature...must succeed on a Dexterity saving throw...

Suppose a sorcerer is granted advantage from being invisible and casts acid splash. RAW seems to indicate that the spell is not an attack roll and thus does not grant advantage, but RAI might imply that your dexterity roll should be at a disadvantage from not knowing you are being attacked. Is the saving throw rolled at a disadvantage?

Answer 1

No, the saving throw is not rolled at disadvantage.

As you have noticed, the spell does not involve making an attack roll. (SRD p. 102)

Saving Throws
Many spells specify that a target can make a saving throw to avoid some or all of a spell’s effects. The spell specifies the ability that the target uses for the save and what happens on a success or failure. The DC to resist one of your spells equals 8 + your spellcasting ability modifier + your proficiency bonus + any special modifiers.

Attack Rolls
Some spells require the caster to make an attack roll to determine whether the spell effect hits the intended target. Your attack bonus with a spell attack equals your spellcasting ability modifier + your proficiency bonus.

Answer 2

No

There's a specific situation where being hidden when casting a spell imposes a disadvantage on the target's saving throw. It is the "Magical Ambush" class feature of the 9th level arcane trickster, PHB pg 98.

It would be game breaking (or at least unfair to Rogues) to apply such an exclusive class feature to any caster in the same situation.