Unchecking "Custom filter" has no effect if custom filter has been entered on Edit Search Page

A small but irritating bug, easy to reproduce:

1. Go to any search page edit form (for example, admin/config/search/apachesolr/search-pages/core_search/edit).
2. Check Custom filter. This reveals a text field labeled Custom filters.
3. Enter a filter, for example entity_type:article
4. Uncheck Custom filter. The text field below disappears.
5. Click on Save configuration.

This UI behavior leads one to expect that the filter will not apply on the search page, but visit the search page and you will see that it does.

Edit the search page again, and you will see that Custom filter is checked and the text field the filter you just entered.

To actually delete the filter you counter-intuitively have to BOTH uncheck Custom filter AND remove the text filter.