That is search, not a filter.
(*) To be fair, it is quite annoying how Search works at the moment, since changing
a dropdown for a search column will clear the search string :(
At the moment of writing in the search field, the user might decide to search some other column.
Or some other. Or other...
What I liked in Jam v4 was the ability to search columns by merely clicking on any column.
Search even had a badge sign (?) with this tip:
'To select a search field hold Ctrl key and click on the corresponding column of the table.'
This was way faster than navigating through the dropdown list. No idea why was this approach dumped.
So what to do for (*)?
First, fork Jam. Why? Because this will not go into the main branch before acceptance.
Find jam.js line 921 and comment out this on your branch:
search.focus();
search.val('');
Same in jam.min.js, find, comment out.
Hence, the (*) is now fixed and you can write a solution for clearing the search field,
and contribute to the project in a similar way.
Cheers
D.