The Shadow's Strength Drain attack states:
Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target’s Strength score is reduced by 1d4. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
For a character that is in their true form this effect is straight forward, it reduces the strength score of the character's true form.
There is an ambiguity over which form the strength drain will apply to if it is applied to a character that is in some form other than their true form.
The most common way for this to occur is if the character is under the effect of Wild Shape, Shapechange, Polymorph or True Polymorph, as these change the character into another creature.
Let's say a Wild Shaped character is affected by the Strength Drain attack while in their beast form. Does the Strength Drain carry over to their true form when they drop their Wild Shape, or does it only affect the specific instance of Wild Shape?
Similar questions also arise for the Shapechange, Polymorph and True Polymorph spells.