I talked to support about this.
I used the 'include words' filter on a batch over over 2500 reviews, and it resultated in only 2 reviews while there are many more reviews that meet the criteria.
Then support explained the widget only captures the last 100 reviews and applies the filter to them, resulting in less then 100 reviews being shown.
It would make more sense to apply the filter first and then limit the result to 100.
Note: I tried to use this workaround to find get reviews in a specific language. The same would be true if you use the filter to find reviews for a specific product.