Clear button returns posts from other categories

Hello :
I have different type of posts on my WordPress website and i have grouped them in many categories. I have created a page and i am trying to use the filter functionality for a limited number of categories.
When i select grouped or default filter, if i search something from the search box and afterwards clear the results, the posts from the specific categories are retrieved correctly, but if i try to use the dropdown filter, or the custom_filter options, after i click the “X” button, it retrieves all of the posts from my site.
I really want to use the dropdown option because is the best option for me.
Do you have a solution for me?
Thanks.