Doesn't work as expected if a default value is set

Problem/Motivation

This doesn't work as expected if there is a default value set for the filter.

Steps to reproduce

1. Create some content with taxonomy applied.
2. Create a view and add the taxonomy filter, and give it a default value.
3. Set up the exposed filters to use BEF, and set the selective option to show only used items.
4. View your view, and see that some taxonomy terms are missing from the filters even if they should have content

I'm attaching two screenshots, the first shows the exposed filters open without a default selected. The second shows the exposed filters with a default selected. You can see that some items are now missing from the available filters.

Note that selecting a default is required if the exposed filter is set to required.