If you type a keyword in the quick filter and delete it by pressing backspace the folder content is still not completely show.

Example: 
1. open folder X in kmail
2. folder shows 1000 mails 
3. type "test" into the quick filter
4. the folder shows 50 entries
5. press backspace until the quick filter is empty
6. the folder shows 50 entries (it should show 1000)

However, if you clear the quick filter by pressing the black x arrow on the right of the quick filter text field, everything works as expected.

If you delete the filter by backspace and type some new keyword, the folder content is also updated to the new filter keyword.

Reproducible: Always