As per the rules of full cover

"A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle"

Full cover gives an explanation about spells being able to reach a target despite full cover.

P204 phb under rule clear path to the target also states.

"If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of the obstruction."

Under Area of effect also page 204

"A spells point of origin extends out ward in straight lines from the point of origin. If no unblocked straight line extends from the the point of origin to the area of effect, that location isn't included in spells area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9."

P241 Fireball (cliff notes)

...to a point you choose within range and then blossoms with a low roar into an explosion of flame....The fire spreads around corners...

With the way fireball is worded it.

Does not move in a straight line.

even if it is placed on the near side of an obstruction will go around corners

Even if there is full cover it will spread around corners.

In short. Take your pick.

The Spell does exactly as it says it does

Its Specefic ruling beats General

The Devolopers won't hide Elephants in Mole hills

As per the rules of full cover:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

Full cover gives an explanation about spells being able to reach a target despite full cover.

Page 204 of the PHB under rule clear path to the target also states.

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of the obstruction."

Under Area of effect also page 204

A spells point of origin extends out ward in straight lines from the point of origin. If no unblocked straight line extends from the the point of origin to the area of effect, that location isn't included in spells area. To block one of these imaginary lines, an obstruction must provide total cover, as explained in chapter 9.

P241 Fireball (cliff notes)

...to a point you choose within range and then blossoms with a low roar into an explosion of flame....The fire spreads around corners...

With the way fireball is worded it.

• Does not move in a straight line.
• even if it is placed on the near side of an obstruction will go around corners
• Even if there is full cover it will spread around corners.

In short. Take your pick.

• The Spell does exactly as it says it does

• Its Specific ruling beats General

• The Developers won't hide Elephants in Mole hills