It is a condition that the ear-drops reach the body cavity.
What you are referring to is a sound observation, based on modern-day medical evidences. Classically the scholars mention the ear as being an invalidating entry point, from which a substance would reach the body cavity. However, modern-day medicine negates any such connection, unless there is a hole in the eardrum.
As such ear-drops would not invalidate the fast unless one has a hole in the eardrum.
However, as a precaution one should avoid using ear-drops while fasting.
[Rafi Usmani, Dawabit al-Mufattirat]
