Selected Facet value is not applied to search filter after toggling another Facet

**Describe the bug 🐛**
When having two Facets that are filterable, the state of a selected facet is lost when selecting another facet.

**To Reproduce 🔍**
I followed the steps in the [getting started example](https://www.algolia.com/doc/guides/building-search-ui/getting-started/flutter/) exactly. With this setup, the following steps show the issue

- Select a value of Facet A and apply filter
- Select a value of Facet B and apply filter
- Unselect the selected value of Facet B and apply filter
-> Found results are completely unfiltered, filter state of Facet A is lost

Note, the checkbox of the selected value for Facet A is still checked.

**Expected behavior 💭**
After the above steps, it should still filter based on the selected value of Facet A

**Environment:**

- OS: tested on iOS and Macos
- Library Version 0.5.0

Start by reproducing the steps in the linked Flutter getting started example with two filterable facets, then trace how facet filter state changes when the second facet is selected and cleared. Done means the selected value from Facet A remains applied and its checked state matches the filtered results after Facet B is removed.

Written by the indexing model from the issue text.