Does a Duergar Spy's Enlarge action double its damage dice from its Sneak Attack trait?

Question

In the module The Forge of Fury (tiny spoiler) there is a Duergar Spy creature. Its statblock is on p. 234 of Tales from the Yawning Portal.

I am interested in the way two of its features interact with each other, Enlarge and Sneak Attack. If the Spy qualifies for a Sneak Attack while Enlarged, are the damage dice added from the sneak attack doubled?

The duergar spy's Enlarge action says:

For 1 minute, the spy magically increases in size [...] While enlarged, the spy is Large, doubles her damage dice on Strength-based weapon attacks, and makes Strength checks and saving throws with advantage.

Its Sneak Attack trait says:

Once per turn, the spy can deal an extra 7 (2d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally that isn't incapacitated and the spy doesn't have disadvantage on the attack roll.

The spy has a Shortsword attack. A Shortsword is normally a finesse weapon, meaning one can use either Str or Dex for to hit and damage. The stat block suggests the Spy uses Dex, but explicitly applies Enlarge to it (the spy has a Str mod of +0 and a Dex mod of +3).

Shortsword. Melee Weapon Attack: +5 to hit. Hit: 6 (1d6+3) piercing damage, or 10 (2d6+3) while enlarged.

Answer 1

Only if they use Strength to attack

With the way those abilities are worded, yes you can double the Sneak Attack damage while enlarged. But only if you choose to use Strength for the attack roll instead of Dexterity.

The enlarged Strength-based Sneak Attack would then be:

Melee Weapon Attack: +2 to hit. Hit: 7 (2d6+0) piercing damage + 14 (4d6) damage from Sneak Attack

If they attacked with Dexterity, their attack while enlarged would be the same as if they were not enlarged.

The Duergar Spy's version of Sneak Attack is actually simplified from the rogue's class feature, and does not have the finesse weapon restriction by RAW (though most DMs would apply it). So it is technically possible for them to Sneak Attack with larger Strength-based weapons, though they don't have one in their stat block.

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Inconsistent Stat Block

As you point out, there is an inconsistency in the stat block for the Duergar Spy. It lists enlarged damage as doubled, but is still using its Dexterity modifier for the attack. By RAW of the Enlarge ability, this is a mistake.

Enlarge's damage increase should only apply to attacks made with Strength, and the rules for damage rolls state:

When attack with a weapon, you add your ability modifier - the same modifier used for the attack roll - to the damage.

If we ignore the inconsistency and play the stat block as written (that Dexterity can be applied to enlarged attacks), then yes, there is no reason Sneak Attack's damage would not also double. The final roll would be:

Melee Weapon Attack: +5 to hit. Hit: 10 (2d6+3) piercing damage + 14 (4d6) damage from Sneak Attack