The Shadow Force Drain attack says:
Force drainage. Melee weapon attack: +4 to hit, get to 5 feet, a creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4. The target dies if it reduces its Force to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.
For a character that is in its true form, this effect is simple, it reduces the strength score of the character's true form.
There is an ambiguity about how force drainage will be applied if it is applied to a character that is in a different form to its true form.
The most common way for this to happen is if the character is under the effect of Wild Form, Shape Change, Polymorph or True Polymorph, since they change the character to another creature.
Let's say that a character with a wild form is affected by the Force Drain attack while in his beast form. Does Force Drain move to its true form when it releases its Wild Form, or does it only affect the specific instance of Wild Form?
Similar questions also arise for the Shape Change, Polymorph and True Polymorph spells.