Uncheck All does not fully remove table filter

I am using Pat’s solution to get multi-select table filters that use a model as a data source to display a check list instead of the search/type ahead that shows up by default.

(As shown here: https://community.skuid.com/skuid/topics/multi-select-filter-option-for-non-picklist-fields)

The filter works great when selecting one or more options. But note that the where clause for region includes ‘’ (empty string).

When “Uncheck All” is used, the where clause for region is still there with only the empty string.

“Uncheck All” a second time to get the expected result (where clause for Region is removed)

Why does “Uncheck All” not remove the where clause for region the first time? I have a filter based on User Roles on the same table that is configured the exact same way and it works perfectly.