Best way to find few documents, (less than 0.01% ) with unusual data in one field using index

Question

My app works with MongoDB using mongo_dart.dart version 0.7.4+1 .

One of the collections has about 60K documents.

One of the fields in this collection, X, can be empty or contains text.

Almost all the queries search for Docs with X=empty.

I created a Partial Filter Expression, but its not used in Explain Plan

partialFilterExpression:{"X":""}

Why the partial index not used?

Can I create an index for the Docs with x=empty?

What is the fast way?